Destroy xwidgets when destroying frames

Destroy xwidgets when destroying frames

* src/xterm.c (x_free_frame_resources): Make sure to kill xwidget views.
* src/xwidget.c (Fmake_xwidget): Attach damage event signal.
(offscreen_damage_event): Operate on xwidgets and not individual
views.
(xwidget_init_view): Don't attach damage event signal here.
(Fdelete_xwidget_view): Destroy window correctly and stop removing
damage event signal.
(kill_frame_xwidget_views): New function.
* src/xwidget.c (kill_frame_xwidget_views): New function.