keep playback context with `TransferPlayback`

[eladyn]

I believe this behavior could be improved, but my bigger concern is what happens even when I successfully invoke TransferPlayback and the org.mpris.MediaPlayer2.spotifyd.instance<PID> interface is exposed — spotifyd loses its playback context. As a result, I am unable to play anything until I manually intervene via the web interface or another device. Ideally, after transferring playback, invoking the PlayPause method should resume playback from the point where the disconnection occurred.

Thank you for looking into this!

Originally posted by @soulseller in #1317 (comment)

This basically includes two separate items:

• automatically transfer playback on reconnection
• restore playback context when calling TransferPlayback