On some setups (mostly flatpak) selection-mode can be triggered on
launch when show_all is called in AlbumsView.
Indeed, if at least one AlbumCover has been loaded before the show_all
call, its check button will become visible. Because of a property
binding, this will change the AlbumCover selection-mode property to
True. This selection-mode change is then propagated up to the Window.

Fix the issue by removing the show_all call and making AlbumsView
visible by default.