layout: new file handling shell layout

Remove ShellGlobal's monitor-related methods, and have
Main.layoutManager provide that information instead. Move
Main._relayout() to LayoutManager, and have other objects connect to
the layout manager's 'monitors-changed' signal to know when the screen
geometry has changed.

https://bugzilla.gnome.org/show_bug.cgi?id=636963

js/ui/windowManager.js

@@ -180,7 +180,7 @@ WindowManager.prototype = {
          */
         this._minimizing.push(actor);
 
-        let primary = global.get_primary_monitor();
+        let primary = Main.layoutManager.primaryMonitor;
         let xDest = primary.x;
         if (St.Widget.get_default_direction() == St.TextDirection.RTL)
             xDest += primary.width;