workspacesView: Return from overview when tapping on an empty workspace

This works for pointers and touch on X11, there is however no pointer
emulation on evdev, so touch triggers ::clicked with button==0 which
is ignored.

https://bugzilla.gnome.org/show_bug.cgi?id=756748

js/ui/workspacesView.js

@@ -421,7 +421,7 @@ const WorkspacesDisplay = new Lang.Class({
             // Only switch to the workspace when there's no application
             // windows open. The problem is that it's too easy to miss
             // an app window and get the wrong one focused.
-            if (action.get_button() == 1 &&
+            if ((action.get_button() == 1 || action.get_button() == 0) &&
                 this._getPrimaryView().getActiveWorkspace().isEmpty())
                 Main.overview.hide();
         }));