Implement smarter detection of windowing environments.

This patch centralises the loading of windowing environments, in order
to ensure that by default only a single module is loaded, rather than
the current code which can load multiple potentially incompatible
modules; and in order to provide the user with more control over the
loading of windowing modules.

The patch introduces a new str-list option `windowing_modules` which
defines an ordered list of windowing modules to attempt to load. Modules
are loaded in the order specified in the list until a module loads
without error, at which point the process finishes.

When loaded each windowing module tests the environment to determine
whether it should load (e.g. the tmux module tests to see if it's being
run within a tmux session), and if it determines that it should then it
completes its loading without error. If it doesn't detect an appropriate
environment then it returns an error, and the module loading logic tries
the next module.

The user can override the default `windowing_modules` list to specify
their preferred modules (i.e. they can put kitty ahead of tmux if that's
their preference, or they can leave out the x11 modules alltogether). In
addition, if the `windowing_modules` option is an empty list this
bypasses the environment detection logic completely, and allows the
modules to be loaded manually - this allows a user to replace the
windowing module loading logic with their own manual set up.

rc/windowing/detection.kak

@@ -0,0 +1,41 @@
+# Attempt to detect the windowing environment we're operating in
+#
+# We try to load modules from the windowing_modules str-list option in order,
+# stopping when one of the modules loads successfully. This ensures that only
+# a single module is loaded by default.
+#
+# On load each module must attempt to detect the environment it's appropriate
+# for, and if the environment isn't appropriate it must fail with an error.
+# In addition, each module must check for the length of the windowing_modules
+# str-list option defined below, and must /not/ check for an appropriate
+# environment if the list is empty. An example of this test:
+#
+# evaluate-commands %sh{
+#     [ -z "${kak_opt_windowing_modules}" ] || [ -n "$TMUX" ] || echo 'fail tmux not detected'
+# }
+#
+# Each module is expected to define at least two aliases:
+#  * terminal - create a new terminal with sensible defaults
+#  * focus - focus the specified client, defaulting to the current client
+#
+
+declare-option -docstring \
+"Ordered list of windowing modules to try and load. An empty list disables
+both automatic module loading and environment detection, enabling complete
+manual control of the module loading." \
+str-list windowing_modules 'tmux' 'screen' 'kitty' 'iterm' 'x11'
+
+hook -group windowing global KakBegin .* %{
+
+    evaluate-commands %sh{
+        set -- ${kak_opt_windowing_modules}
+        if [ $# -gt 0 ]; then
+            echo 'try %{ '
+            while [ $# -gt 1 ]; do
+                echo "require-module ${1} } catch %{ "
+                shift
+            done
+            echo "require-module ${1} }"
+        fi
+    }
+}

rc/windowing/iterm.kak

@@ -1,16 +1,13 @@
 # https://www.iterm2.com
 # ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
 
-## The default behaviour for the `terminal` command is to open a vertical pane in
-## an iTerm session if not in a tmux session.
-hook global KakBegin .* %sh{
-    if [ "$TERM_PROGRAM" = "iTerm.app" ] && [ -z "$TMUX" ]; then
-        echo "require-module iterm"
-    fi
-}
-
 provide-module iterm %{
 
+# ensure that we're running on iTerm
+evaluate-commands %sh{
+    [-z "${kak_opt_windowing_modules}" ] || [ "$TERM_PROGRAM" = "iTerm.app" ] || echo 'fail iTerm not detected'
+}
+
 define-command -hidden -params 2.. iterm-terminal-split-impl %{
     nop %sh{
         direction="$1"

rc/windowing/kitty.kak

@@ -1,12 +1,13 @@
-
-hook -group kitty-hooks global KakBegin .* %sh{
-    if [ "$TERM" = "xterm-kitty" ] && [ -z "$TMUX" ]; then
-        echo "require-module kitty"
-    fi
-}
+# https://sw.kovidgoyal.net/kitty/index.html
+# ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
 
 provide-module kitty %{
 
+# ensure that we're running on kitty
+evaluate-commands %sh{
+    [-z "${kak_opt_windowing_modules}" ] || [ "$TERM" = "xterm-kitty" ] || echo 'fail Kitty not detected'
+}
+
 declare-option -docstring %{window type that kitty creates on new and repl calls (kitty|os)} str kitty_window_type kitty
 
 define-command kitty-terminal -params 1.. -shell-completion -docstring '

rc/windowing/screen.kak

@@ -1,13 +1,14 @@
 # http://gnu.org/software/screen/
 # ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
 
-hook -group GNUscreen global KakBegin .* %sh{
-    [ -z "${STY}" ] && exit
-    echo "require-module screen"
-}
 
 provide-module screen %{
 
+# ensure that we're running under screen
+evaluate-commands %sh{
+    [-z "${kak_opt_windowing_modules}" ] || [ -n "$STY" ] || echo 'fail screen not detected'
+}
+
 define-command screen-terminal-impl -hidden -params 3.. %{
     nop %sh{
         tty="$(ps -o tty ${kak_client_pid} | tail -n 1)"

rc/windowing/tmux.kak

@@ -1,14 +1,13 @@
 # http://tmux.github.io/
 # ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
 
-hook global KakBegin .* %sh{
-    if [ -n "$TMUX" ]; then
-        echo "require-module tmux"
-    fi
-}
-
 provide-module tmux %{
 
+# ensure we're running under tmux
+evaluate-commands %sh{
+    [ -z "${kak_opt_windowing_modules}" ] || [ -n "$TMUX" ] || echo 'fail tmux not detected'
+}
+
 define-command -hidden -params 2.. tmux-terminal-impl %{
     evaluate-commands %sh{
         tmux=${kak_client_env_TMUX:-$TMUX}

rc/windowing/x11.kak

@@ -1,13 +1,12 @@
 # x11
 
-hook global KakBegin .* %sh{
-    if [ -n "$DISPLAY" ] && [ -z "$TMUX" ]; then
-        echo "require-module x11"
-    fi
-}
-
 provide-module x11 %{
 
+# ensure that we're running in the right environment
+evaluate-commands %sh{
+    [-z "${kak_opt_windowing_modules}" ] || [ -n "$DISPLAY" ] || echo 'fail DISPLAY is not set'
+}
+
 # termcmd should be set such as the next argument is the whole
 # command line to execute
 declare-option -docstring %{shell command run to spawn a new terminal