ime: Add builtin handling of dead keys

ime: Add builtin handling of dead keys

The IME input method has been both ignoring keypresses of
non-spacing characters (ditching these as non displayable),
and not letting IME do anything about those.

Even though, the sparse documentation on IMM/IME seems to
hint that applications can't pipe non-spacing characters to
the input method manager, and experimentation shown that
those characters are indeed handled differently than how
it'd be expected.

Then, add basic handling of dead keys on the IME input method
itself , as it's not mutually exclusive with regular keymaps
with dead keys.

https://bugzilla.gnome.org/show_bug.cgi?id=704937