alt-tab: Do not hardcode ALT modifier

While we allow for arbitrary modifiers in keybindings, both the
alt-tab and ctrl-alt-tab popups close when ALT is not present in
the modifier mask, resulting in ALT being de-facto hardcoded.
Instead, pass the actual modifier mask when invoking the popups.

https://bugzilla.gnome.org/show_bug.cgi?id=645200

js/ui/main.js

@@ -607,7 +607,8 @@ function _globalKeyPressHandler(actor, event) {
     }
 
     if (action == Meta.KeyBindingAction.SWITCH_PANELS) {
-        ctrlAltTabManager.popup(modifierState & Clutter.ModifierType.SHIFT_MASK);
+        ctrlAltTabManager.popup(modifierState & Clutter.ModifierType.SHIFT_MASK,
+                                modifierState);
         return true;
     }