WorkspaceSwitcherPopup: allow input events to pass through

There's no particular reason for this actor to be reactive and thus
preventing input events to reach actors underneath, e.g. quickly
clicking on something while the popup isn't yet finished animating
out.

https://bugzilla.gnome.org/show_bug.cgi?id=770382

js/ui/workspaceSwitcherPopup.js

@@ -19,8 +19,7 @@ const WorkspaceSwitcherPopup = new Lang.Class({
     Name: 'WorkspaceSwitcherPopup',
 
     _init : function() {
-        this.actor = new St.Widget({ reactive: true,
-                                     x: 0,
+        this.actor = new St.Widget({ x: 0,
                                      y: 0,
                                      width: global.screen_width,
                                      height: global.screen_height,