API: gdk: Change get_drag_window() API

The previous function gdk_drag_get_protocol_for_display() took native
window handles, so it had to be changed. Because it didn't do what it
was named to do (it didn't return a protocol even though it was named
get_protocol) and because it doesn't operate on the display anymore but
on the actual window, it's now called gdk_window_get_drag_protocol().

gdk/gdkwindow.h

@@ -780,6 +780,10 @@ void          gdk_window_set_opacity     (GdkWindow       *window,
                                           gdouble          opacity);
 void          gdk_window_register_dnd    (GdkWindow       *window);
 
+GdkDragProtocol
+              gdk_window_get_drag_protocol(GdkWindow      *window,
+                                          GdkWindow      **target);
+
 void gdk_window_begin_resize_drag (GdkWindow     *window,
                                    GdkWindowEdge  edge,
                                    gint           button,