Move logical monitor neighbor API into monitor manager

Move the last piece of monitor grid getter API to the monitor manager
away from MetaScreen. The public facing API are still there, but are
thin wrappers around the MetaMonitorManager API.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/wayland/meta-window-wayland.c

@@ -339,9 +339,9 @@ meta_window_wayland_update_main_monitor (MetaWindow *window)
   MetaMonitorManager *monitor_manager =
     meta_backend_get_monitor_manager (backend);
   MetaWindow *toplevel_window;
-  const MetaLogicalMonitor *from;
-  const MetaLogicalMonitor *to;
-  const MetaLogicalMonitor *scaled_new;
+  MetaLogicalMonitor *from;
+  MetaLogicalMonitor *to;
+  MetaLogicalMonitor *scaled_new;
   float scale;
   MetaRectangle rect;