Handle resolution changes

Added signal 'screen-size-changed' to ShellGlobal.
Connect to this signal in main.js and run the _relayout() method.
If Overview or calendar are visible when this signal emit, they will be hiding.
https://bugzilla.gnome.org/show_bug.cgi?id=584526

js/ui/overview.js

@@ -158,6 +158,8 @@ Overview.prototype = {
     relayout: function () {
         let primary = global.get_primary_monitor();
 
+        this._recalculateGridSizes();
+
         this._group.set_position(primary.x, primary.y);
 
         let contentY = Panel.PANEL_HEIGHT;