Method for adding custom music

[dreamscached]

There is a use case for external code to add songs to the music selector menu (example how it's currently done in a hacky way)

However, it's impossible to add music to the list in a non-hacky way, as __paginate is protected and name-mangled.

This PR addresses that and adds a simple method for adding a song to the list (optionally, with setting the corresponding PM for custom music)

[Booplicate]

I kinda want to refactor the music system to allow for a more advanced player.
Furthermore I think we should be more careful with what we expose as public API, we have to live with those things basically forever, which means we can't easily change our systems.

[dreamscached]

If we happen to change the system, I still think we should have a way to externally add music to it; it doesn't feel right to put submod custom music to the player's music folder, instead it could be added from submod folder and played from there directly.