windowsNavigator: do nothing when viewtab is not active

The keybindings should only kick-in when the "windows" tab is active,
otherwise it is confusing and could conflict with other combinations.

extensions/windowsNavigator/extension.js

@@ -139,6 +139,9 @@ function enable() {
     workViewInjections['_onKeyRelease'] = undefined;
 
     WorkspacesView.WorkspacesView.prototype._onKeyPress = function(s, o) {
+        if(Main.overview._viewSelector._activeTab.id != 'windows')
+            return false;
+
         if ((o.get_key_symbol() == Clutter.KEY_Alt_L ||
              o.get_key_symbol() == Clutter.KEY_Alt_R)
             && !this._pickWorkspace) {