window: Fix album view selection by the stack switcher on launch
The first view added to the stack is the EmptyView which is not visible on launch. Then, AlbumsView is added and becomes the visible_child by default because it is visible. However, it is not selected by the the stack switcher because the "notify::visible-changed" signal is received before the child is added to the stack switcher. This issue is fixed by adding a "visible-child" notification. This way, the stack switcher selected the AlbumView on launch.