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.

Since there is going to be an actual IM reset anyways, it does
no longer make sense to try to preserve the old priv->need_im_reset
status during commit handling.

Fixes: 52ac71b972 ("gtktextview: Shuffle the places doing IM reset")
Closes: https://gitlab.gnome.org/GNOME/gtk/-/issues/5133