Exclude media streams from MediaStreamSource and MediaPlaybackItem

[brabebhin]

This adds a new interface that can be used by applications to decide if a stream should be included in the MediaStreamSource or MediaPlaybackItem.

Users need to implement the interface and pass it in the MediaSourceConfig.General options.

This should enable the scenario described on #425

Open points:

1. I don't really like the use of the word "filter" there, not to be confused with effects or media filters. Haven't found a better wording yet.
2. Should we support async calls as well?