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/modalDialog.js

@@ -149,7 +149,7 @@ ModalDialog.prototype = {
     },
 
     _fadeOpen: function() {
-        let monitor = global.get_focus_monitor();
+        let monitor = Main.layoutManager.focusMonitor;
 
         this._backgroundBin.set_position(monitor.x, monitor.y);
         this._backgroundBin.set_size(monitor.width, monitor.height);