Automatically add songOrder to playlists

[daladim]

This fixes #117

When adding songs to a playlist, let's automatically assign it a sortOrder so that it appears at the end of the playlist.

This does not prevent to re-order songs after they have been added.

[daladim]

I was not sure how to implement PlaylistSongJoinDao.appendSongs() using a simple, single SQLite query. Since it was used at two places, I used a simple iteration on all songs instead.

We could probably find some magic SQLite query that does this for us, and which would be more performant than iterating ourselves, but I don't think users will often add millions of songs to a playlist, so maybe this is fine this way.

[timusus]

Hey @daladim, would you mind reviewing #119

It should solve the problem in a similar fashion, but without the need for the SQL query.

I slightly prefer that approach, as it doesn't require us to query the database song count for every song insert. It's possibly slightly more efficient.