gtk: Fix race between queue_draw and destroy

gtk: Fix race between queue_draw and destroy

In GTK dispose can be called before the last ref is reached. This
happens when you close the container window. The dispose will be
explicitly called, and destroyed notify will be fired. This patch
fixes this race by properly tracking the widget state.

In the sink, we now set the widget pointer to NULL, so the widget
will properly get created again if you set your pipeline to NULL
state after the widget was destroy, and set it back to PLAYING.

https://bugzilla.gnome.org/show_bug.cgi?id=751104