Pasting a lot of text could cause a deadlock between writing
to the input side of the pty and consuming the output side.
Making the writer/input side non-blocking resolves this.
Dead key processing respects the
`send_composed_key_when_left_alt_is_pressed` and
`send_composed_key_when_right_alt_is_pressed` options.
See doc changes included in this commit for more info.
refs: https://github.com/wez/wezterm/issues/410
This commit changes mouse-based selection and middle click to use the
PrimarySelection.
CTRL-SHIFT-{C,V} use Clipboard.
{SHIFT,CTRL}-Insert use PrimarySelection.
`CompleteSelection` and `CompleteSelectionOrOpenLinkAtMouseCursor` now
require a parameter to specify the destination clipboard.
Removed the `default_clipboard_XXX` options added in
8dad34fa61 in favor of just explicitly
assigning the key/mouse bindings.
closes: #417
