Use GDK's current window tracking when synthesizing events in GTK+

Add gdk_device_get_last_event_window(), and use to implement the window
tracking we need for synthesizing crossing events for sensitivity changes
and gtk grabs, rather than keeping the information in qdata and updating
it based when GTK+ gets events.

https://bugzilla.gnome.org/show_bug.cgi?id=726187

docs/reference/gdk/gdk3-sections.txt

@@ -736,6 +736,7 @@ GdkTimeCoord
 gdk_device_get_axis
 gdk_device_list_axes
 gdk_device_get_axis_value
+gdk_device_get_last_event_window
 
 <SUBSECTION Standard>
 GDK_TYPE_AXIS_USE