gtkimcontextwayland: Refactor handling of client updates
Currently, the wayland IM context sends zwp_text_input_v3.commit from
a number of places, and some of them with partial data. In order to
make client state updates "atomic" and complete, make the communication
happen over an unified notify_im_change() function that happens on
a narrower set of circumstances:
- The GtkIMContext is reset
- The GtkIMContext is just focused
- The gesture to invoke the OSK is triggered
- The IM context is reacting to changes coming from the compositor
Notably, setting the cursor location or the surrounding text do not try
to commit state on their own, and now will be flushed with the corresponding
IM update or reset. But also, these requests won't be prevented from
happening individually on serial mismatch, instead it will be the whole
state commit which is held off.
With these changes in place, all client-side updates are notified
atomically to the compositor under a single .commit request.
Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/5106
Fixes: https://gitlab.gnome.org/GNOME/gtk/-/issues/5105