playbin3: crossfading support
Submitted by olivier dufour
A needed features by a lot of player is to add cross fading support in playbin2.
Playbin2 ever support gapless. So this is the only one features missing to have a full features pipeline for audio player.
Currently, it can be done by building a dedicaced pipeline with an adder pad to mix stream. But we can not mix 2 plabin2 in an adder. So maybe an easier solution is to allow to link n playbin2 in adder.