switcherPopup: Add a return value to _keyPressHandler

This allows us to handle keybindings that use Escape instead of just
dismissing the popup unconditionally when we see an Escape press.

https://bugzilla.gnome.org/show_bug.cgi?id=730739

js/ui/switcherPopup.js

@@ -190,10 +190,11 @@ const SwitcherPopup = new Lang.Class({
 
         this._disableHover();
 
+        if (this._keyPressHandler(keysym, backwards, action) != Clutter.EVENT_PROPAGATE)
+            return Clutter.EVENT_STOP;
+
         if (keysym == Clutter.Escape)
             this.destroy();
-        else
-            this._keyPressHandler(keysym, backwards, action);
 
         return Clutter.EVENT_STOP;
     },