Specify caps and events relation to stream more strictly

[mat-hek]

Proposal:

• Caps and sync events should be considered part of the stream and therefore transferred only while playing and cleaned up upon prepare (Buffers and demands should be cleaned up after playback -> prepare #55), along with all other parts of the stream and information derived from the stream (current caps, current demand size etc)
• To allow earlier preparation of elements when caps are available before playing, CapsPredictions can be sent. Caps, however, need to be sent separately and not induced from predictions.
• Async events and caps predictions are not part of the stream.
• What is part of the stream is transferred keeping the order.
• We need to decide whether caps should be sticky or not.

@bblaszkow06 @mspanc