playlist-dialog: don't show static playlists

Static playlists are managed by GNOME Music and shouldn't
be exposed as available playlists in the playlist dialog,
even if adding to them works.

Fix that by adding a new method to check whether a playlist
is static or not, and teaching the playlists dialog to not
add static playlists.

https://bugzilla.gnome.org/show_bug.cgi?id=744834