API change: stream selection to player

[katajakasa]

Stream selection should be done in player, not in source.

• Source should be a pure representation of source data
• Player should decide on what to play and how to play it.
• Player should have stream selection in create functions and during-playback selector functions.

[katajakasa]

Decided not to do this for v1, due to attempt to keep API as simple as possible.

Out of scope for v1, something maybe for v2.

[katajakasa]

Experimental branch at #45