gtkwidget: Ensure unrealization during event dispatching cancels gestures

gtkwidget: Ensure unrealization during event dispatching cancels gestures

We use to rely on grab broken events for most of the event sequence
lifetime, this breaks though on GDK_BUTTON_RELEASE/GDK_TOUCH_END, as there's
no longer a grab at that time.

For these cases (and all others where there's destroy/unrealize calls
involved during event dispatching), catch this on the late
WIDGET_REALIZED_FOR_EVENT calls on widget event handling functions.

https://bugzilla.gnome.org/show_bug.cgi?id=754098