Move manipulation of the X stack to MetaStackTracker

Since MetaStackTracker is the code that knows about the current X stacking order
and the relationship between X windows and Wayland windows, it's cleaner to
encapsulate stack manipulation in MetaStackTracker rather than have the calling
code make the X calls and only call into MetaStackTracker to inform it about
the changes.

https://bugzilla.gnome.org/show_bug.cgi?id=736559

src/core/screen.c

@@ -490,10 +490,8 @@ create_guard_window (Display *xdisplay, MetaScreen *screen)
                                  guard_window,
                                  create_serial);
 
-  meta_stack_tracker_record_lower (screen->stack_tracker,
-                                   guard_window,
-                                   XNextRequest (xdisplay));
-  XLowerWindow (xdisplay, guard_window);
+  meta_stack_tracker_lower (screen->stack_tracker,
+                            guard_window);
   XMapWindow (xdisplay, guard_window);
   return guard_window;
 }