Merge lp://qastaging/~jderose/novacut/audio into lp://qastaging/novacut

the builder functions now have this call signature:

build_func(builder, doc, offset=0)

And these fuctions now add elements to the gnlcomposition themselves, and returns a durtation for this node, or at least the duration that should be added the offset to compute the offset the next node in the sequence will use.

Importantly, this API change allows the builder functions to add multiple slices to the composition. This is needed for popper audio support and for any of the composite node-types we'll eventually have.

I also wrongly thought you could put both audio and video slices in the same gnlcompositon, but that's no the case. So now the Builder has one composition for audio, one for video.