API: widget: Add gtk_widget_set_clip() API

And handle the fact that drawing bounds are now handled by this API and
the corresponding gtk_widget_get_clip().

Also add _gtk_widget_supports_clip() function to check if a widget has
been ported to the new world.

docs/reference/gtk/gtk3-sections.txt

@@ -5475,6 +5475,8 @@ gtk_widget_get_allocated_height
 gtk_widget_get_allocation
 gtk_widget_set_allocation
 gtk_widget_get_allocated_baseline
+gtk_widget_get_clip
+gtk_widget_set_clip
 gtk_widget_get_app_paintable
 gtk_widget_get_can_default
 gtk_widget_set_can_default