inputMethod: Use update-preedit-with-mode signal

inputMethod: Use update-preedit-with-mode signal

This signal announces the preferred commit mode of the preedit text when
the input context is reset. Keep this mode around, and ensure to honor this
mode (e.g. maybe commit the preedit string) whenever the input method would
be reset.

This is delegated to the internal layers, so propagate this mode via
clutter_input_method_set_preedit_text().

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1929>