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/panel.js

@@ -398,6 +398,7 @@ Panel.prototype = {
 
         this._clock = new St.Label();
         clockButton.set_child(this._clock);
+        this._clockButton = clockButton;
 
         this._calendarPopup = null;
 
@@ -485,6 +486,13 @@ Panel.prototype = {
         this._updateClock();
     },
 
+    hideCalendar: function() {
+        if (this._calendarPopup != null) {
+            this._clockButton.checked = false;
+            this._calendarPopup.actor.hide();
+        }
+    },
+
     startupAnimation: function() {
         this.actor.y = -this.actor.height;
         Tweener.addTween(this.actor,