x11: Add support for _GTK_EDGE_CONSTRAINTS atom

To keep feature parity with the Wayland backend, and
to improve the overall tiling experience with GTK apps,
add the _GTK_EDGE_CONSTRAINTS X11 atom and update it
when necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=751857

src/core/screen.c

@@ -308,6 +308,7 @@ set_supported_hint (MetaScreen *screen)
 
     screen->display->atom__GTK_FRAME_EXTENTS,
     screen->display->atom__GTK_SHOW_WINDOW_MENU,
+    screen->display->atom__GTK_EDGE_CONSTRAINTS,
   };
 
   XChangeProperty (screen->display->xdisplay, screen->xroot,