widgets: Enable selection mode on right-click in ArtistAlbums
view: Enable selection mode on right-click when viewing an album

https://bugzilla.gnome.org/show_bug.cgi?id=728407
view: Make albums list insensitive in Artists view in selection mode

Only the sidebar should be sensitive in Artists view if selection mode
is on to be consistent with the Playlists view.

This prevents any song to be played and have both the selection bar and
player bar to be shown at the same time while in selection mode.

https://bugzilla.gnome.org/show_bug.cgi?id=729521
view: Group songs into albums if source doesn't support queries