layout: Trigger the message tray by downward pressure

A pressure barrier is a barrier that activates after the user pushes
against the bottom of the screen in a short time. Implement this using
the new XInput 2.3 features that provide extended information about
pointer barriers, and use it so that pushing against the bottom of
the screen edge brings up the message tray.

https://bugzilla.gnome.org/show_bug.cgi?id=677215

js/ui/messageTray.js

@@ -1706,10 +1706,13 @@ const MessageTray = new Lang.Class({
         if (currentUserTime != this._trayDwellUserTime)
             return false;
 
+        this.openTray();
+        return false;
+    },
+
+    openTray: function() {
         this._traySummoned = true;
         this._updateState();
-
-        return false;
     },
 
     _onNotificationKeyRelease: function(actor, event) {