PopupMenuManager: Fix child menus

Since commit c84dc6254d, popup menus are closed automatically
when another menu opens (to catch the case where a menu is opened
by keyboard shortcut, which wasn't handled before). However in the
case of child menus, both child and parent are expected to be visible,
so handle this case explicitly.

https://bugzilla.gnome.org/show_bug.cgi?id=699678

js/ui/popupMenu.js

@@ -1875,7 +1875,7 @@ const PopupMenuManager = new Lang.Class({
 
     _onMenuOpenState: function(menu, open) {
         if (open) {
-            if (this.activeMenu)
+            if (this.activeMenu && !this.activeMenu.isChildMenu(menu))
                 this.activeMenu.close(BoxPointer.PopupAnimation.FADE);
             this._grabHelper.grab({ actor: menu.actor, modal: true, focus: menu.sourceActor,
                                     onUngrab: Lang.bind(this, this._closeMenu, menu) });