Fixes#5494
When ibus/fcitx is not running (the GTK "simple" input method is
active), the preedit end event triggers _before_ the commit event. For
both ibus/fcitx, the opposite is true. We were relying on this ordering.
This commit changes the GTK input handling to not rely on this ordering.
Instead, we encode our composing state into the boolean state of whether
a key event is pressed. This happens before ANY input method events are
triggered.
Tested dead key handling on: X11/Wayland, ibus/fcitx/none.
Mitchell Hashimoto