Enable hot corner for triggering the overview mode

Use the 1x1 actor in the top left corner of the screen to enter and leave
the overview mode by just moving the mouse over to it.

No delay is used for triggering the hot corner because a delay significant
enough to allow moving away the mouse to avoid triggering it ruins the desired
flow when triggering the hot corner is actually intended.

The hot corner is not enabled in the full screen mode because the application
or the virtual system might have a hot corner of its own in that place.

js/ui/panel.js

@@ -76,6 +76,22 @@ Panel.prototype = {
 
         box.append(this.button.button, Big.BoxPackFlags.NONE);
 
+        let hotCorner = new Clutter.Rectangle({ width: 1,
+                                                height: 1,
+                                                opacity: 0,
+                                                reactive: true });
+
+        // In addition to being triggered by the mouse enter event, the hot corner
+        // can be triggered by clicking on it. This is useful if the user wants to 
+        // immediately undo the effect of triggering the hot corner once in the 
+        // hot corner.
+        hotCorner.connect('enter-event',
+                           Lang.bind(this, this._onHotCornerTriggered));
+        hotCorner.connect('button-release-event',
+                           Lang.bind(this, this._onHotCornerTriggered));
+
+        box.add_actor(hotCorner);
+
         let statusbox = new Big.Box();
         let statusmenu = this._statusmenu = new Shell.StatusMenu();
         statusmenu.get_icon().hide();
@@ -201,5 +217,10 @@ Panel.prototype = {
         this._clock.set_text(displayDate.toLocaleFormat("%a %l:%M %p"));
         Mainloop.timeout_add(msecRemaining, Lang.bind(this, this._updateClock));
         return false;
+    },
+
+    _onHotCornerTriggered : function() {
+        Main.overlay.toggle();
+        return false;
      }
 };