layout: Position important actors above top_window_group

The top_window_group is used by some windows like OpenGL games that want to be shown in fullscreen but don't actually switch to fullscreen mode using our APIs. Since we want modal dialogs, important notifications or the screen keyboard to be visible in this case, put those actors above the top_window_group while keeping the panel underneath it.

Note, that this reverts the changes from 9786b2d0, and also changes the behaviour to quite the opposite of what the top_window_group was added for, see https://bugzilla.gnome.org/show_bug.cgi?id=702338.

Also, since windows in the top_window_group are not the same as fullscreen windows, all notifications will be shown above those windows, not just critical ones. This is especially annoying because most games hold a pointer grab, so the notification can't be dismissed. To fix this we should probably push a modal in case the pointer is grabbed when showing a banner.

Oh, and I just assumed those games don't use our APIs for switching to fullscreen mode, so if that's wrong please correct me!