Previously, the mux layer had no internal understanding of titles other
than the Pane::get_title method to return state from a pane.
Users have asked for ways to explicitly set titles on windows and tabs,
so this commit is a step towards that.
The mux window and tab objects now store a title string.
The terminal layer now emits Alert::WindowTitleChanged when the window
title is changed via eg: OSC 0 or OSC 2.
The mux layer will respond to Alert::WindowTitleChanged by resolving the
window that corresponds to the source pane and amending its title.
The MuxWindow and MuxTab objects now provide accessor methods for the
title.
TabInformation (as used by format-tab-title and format-window-title) now
exposes the underlying window_id as well as tab_title and window_title.
The tab title can be changed via the lua MuxTab type, but there is not
currently an escape sequence associated with this.
The defaults for format-tab-title and format-window-title don't
currently consider these new title strings.
refs: https://github.com/wez/wezterm/issues/1598
