Play show theme music in the background

[pannal]

GHI (If applicable): #2, rewrite of #204

Description:

Plays the current episode's show theme music, or in case of a library-show-visit, the show's theme music.

Adds options for that (off by default) and a volume slider for the theme volume.

Better background music handling approach; PlexPlayer now handles that itself, together with BGMPlayerHandler.

Checklist:

• I have based this PR against the develop branch

[ruuk]

Nice changes. Setting looks good. Bonus points for ListSetting base class 😄.

A couple of things I noticed. The first was that if background music is playing and you are on the home screen, you can't navigate to the header bar. The second (which probably makes the first thing irrelevant) was that we still need to only have BG music playing within the context of a show. So that pretty much means that the only screens that will have BG music will be the show screen and the episodes screen.

So for example if you open a show, BG music. Click a season, BG music continues. Go back to the show, BG music continues. Exit the show, BG music stops.

[pannal]

Implemented all that, as well as proper handling for related shows, e.g.: go to a related show, its theme plays, go back to the show/episode you came from, related show theme stops. The current one also doesn't re-start when coming from a related show, that would make it a bit weird.

Shouldn't the theme music stuff be default on?

[pannal]

Don't merge this yet. When dealing with slower devices or connections, this blocks the interface. I at least need to look into deferring the playback. If that's how it has to be, that's OK with me, if it's by design and not necessarily fixable.

But I have to try.

[pannal]

This should be mergable now.

[pannal]

Rebased onto develop

[pannal]

Playback is deferred, but depending on the platform, starting music playback can still block the interface for a short while - I guess that's Kodi and we can't do anything about that.

[pannal]

Rebased