Relayout on monitor layout changes

Currently we only relayout when the screen size changes, this gets
 the cases where a monitor gets added/removed but not when the primary
 monitor changes.

 We need to relayout on all monitor layout changes.

 Remove ShellGlobal::screen-size-changed signal as it is no longer used, Gdk is
 used to track changes now.

 A ShellGlobal::gdk-screen property is added for this purpose.

https://bugzilla.gnome.org/show_bug.cgi?id=620377

js/ui/main.js

@@ -159,7 +159,7 @@ function start() {
     global.stage.add_actor(background);
     background.lower_bottom();
 
-    global.connect('screen-size-changed', _relayout);
+    global.gdk_screen.connect('monitors-changed', _relayout);
 
     ExtensionSystem.init();
     ExtensionSystem.loadExtensions();