This is my third (!) attempt at implementing localization support.
By leveraging GTK builder to do most of the `gettext` calls, I
can avoid the whole mess about missing symbols on non-glibc platforms.
Added some documentation too for contributors and translators,
just for good measure.
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.
