appIconMenu: Disable sourceActor key events

Enter/space are already taken over by the app icon to activate the
app, and arrow keys interfere with keyboard navigation where icons
are arranged in a grid (namely the app picker).

Also as app icons only create the menu when it is first needed, the
shortcuts only starts to work once a menu has been opened, which is
rather inconsistent and confusing.

It is better to simply disable the key handling altogether for the
app icon menu.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/98

js/ui/appDisplay.js

@@ -1854,6 +1854,11 @@ var AppIconMenu = new Lang.Class({
         Main.uiGroup.add_actor(this.actor);
     },
 
+    // disable parent's handler
+    _onKeyPress(actor, event) {
+        return Clutter.EVENT_PROPAGATE;
+    },
+
     _redisplay() {
         this.removeAll();