grabHelper: Ignore events from On-Screen-Keyboard

GrabHelper automatically releases grabs when the user clicks outside
the grabbed actors. However at least for the message-tray (which is
the only user of grabHelper at the moment), we must ignore any events
from the On-Screen-Keyboard, to prevent the tray from hiding at every
key press.

https://bugzilla.gnome.org/show_bug.cgi?id=683546

js/ui/grabHelper.js

@@ -318,6 +318,9 @@ const GrabHelper = new Lang.Class({
         if (this._isWithinGrabbedActor(event.get_source()))
             return false;
 
+        if (Main.keyboard.shouldTakeEvent(event))
+            return false;
+
         if (button) {
             // If we have a press event, ignore the next event,
             // which should be a release event.