albumsview: Prevent a selection-mode trigger on launch

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.

data/ui/AlbumsView.ui

@@ -2,6 +2,7 @@
 <interface>
   <requires lib="gtk+" version="3.18"/>
   <template class="AlbumsView" parent="GtkStack">
+    <property name="visible">True</property>
     <child>
       <object class="GtkScrolledWindow" id="_scrolled_window">
         <property name="visible">True</property>

gnomemusic/views/albumsview.py

@@ -95,8 +95,6 @@ class AlbumsView(Gtk.Stack):
         self._scrolled_window.props.vadjustment.connect(
             "changed", self._on_vadjustment_changed)
 
-        self.show_all()
-
     def _on_vadjustment_changed(self, adjustment):
         if self._adjustment_timeout_id is not None:
             GLib.source_remove(self._adjustment_timeout_id)