Add music controls to lock screen

[danielnd14]

When you are listening to a playlist in the music app if the elementary OS or the user blocks the session, the music stops. It would be better if it was possible to control the music from the login screen, without being necessary to type in the password again. The music then would be interrupted only if the user wants or if another user logs in.

[cassidyjames]

Thanks for the issue! In the future, please title any issues with a concise summary of the bug or feature request.