layout: Construct the primary monitors's hot corner, too

This cleans up the code considerably, and makes it so that
one path creates all hot corners for all monitors. Why this
wasn't done originally, I have no clue...

The one complication is debouncing if the button and hot corner
are triggered in rapid succession, so we just move this tracking
to the overview.

https://bugzilla.gnome.org/show_bug.cgi?id=663661

js/ui/main.js

@@ -148,8 +148,6 @@ function startSession() {
     else
         screenShield = new ScreenShield.ScreenShieldFallback();
 
-    // The message tray relies on being constructed
-    // after the panel.
     panel = new Panel.Panel();
     messageTray = new MessageTray.MessageTray();
     keyboard = new Keyboard.Keyboard();