Update _NET_CURRENT_DESKTOP on workspace removal

When we delete a workspace before the active workspace, we need
to upate the _NET_CURRENT_DESKTOP since the active workspace index
changes. To do this workspace.c:set_active_space_hint() is moved
to screen.c:meta_screen_set_active_workspace_hint() so that it
can be shared.

https://bugzilla.gnome.org/show_bug.cgi?id=640581

src/core/screen-private.h

@@ -255,4 +255,6 @@ void     meta_screen_workspace_switched (MetaScreen         *screen,
                                          int                 to,
                                          MetaMotionDirection direction);
 
+void meta_screen_set_active_workspace_hint (MetaScreen *screen);
+
 #endif