rtph264pay: Support STAP-A bundling
Add a new property do-aggregate to the H.264 RTP payloader which enables STAP-A aggregation as per RFC-6184.
With aggregation enabled, packets are bundled instead of sent immediately, up until the MTU size.
Bundles also end at access unit boundaries or when packets have to be fragmented.
The property-name is kept generic since it might apply more widely, e.g. STAP-B or MTAP.
This feature is preceded by some extensive refactoring.
Compared to the old patches attached to #434, the refactoring has been split into smaller commits which should be easier to review.