gtktext: Avoid early IM reset on updates

gtktext: Avoid early IM reset on updates

Resetting the IM on IM updates is too eager and indeed the simple
IM context doesn't like that this happens in the middle of dead
key handling.

We however want to reset the IM after actual text buffer changes
(say, a committed string) moved the cursor position, altered the
surrounding text, etc. So that the IM implementation does know to
update its state.

Fixes: 9e29739e66 ("gtktext: Shuffle the places doing IM reset")
Closes: https://gitlab.gnome.org/GNOME/gtk/-/issues/5133