Always store GTK Widget as GtkWidget*.

* src/xwidget.h (struct xwidget): GtkContainer -> GtkWidget for
widgetwindow_osr.
(struct xwidget_view): GtkContainer -> GtkWidget for widgetwindow and
emacswindow.
* src/xwidget.c (Fmake_xwidget): Remove dead code.
* src/xwidget.c: Redraw only the views on damage-event.
* src/xwidget.c (xwidget_init_view): Remove dead code.
* src/xwidget.c: Implement events forwarding with the set_embedder strategy.
* src/xwidget.c (xwidget_init_view): Transfer all events.
* src/xwidget.c (xwidget_init_view): Transfer embedder ownership when the
pointer enter a widget, not on motion.
* src/xwidget.c: Remove old code.
* src/xwidget.c (Fmake_xwidget): Force each widget to be shown.
Merge pull request #14 from daimrod/offscreen_forward_events

Offscreen forward events