Pass through caret position from native GTK input methods

Pass through caret position from native GTK input methods

* src/gtkutil.c (xg_im_context_preedit_changed): Set caret
position appropriately.
* src/xfns.c (xg_widget_key_press_event_cb): Ignore modifier
keys.
(xic_preedit_caret_callback, xic_preedit_draw_callback):
Constrain caret position to the string length as well.