Make consecutive blank clips into a single clip with many repetitions

[Lacrymology]

Right now the blank clip gets repeated as different XMLs and loaded separateldy into melted.

This brings a number of problems, from performance, to the fact that the status reports (e.g.) current and next clips as "blank". Rather that fixing this at the status building step, making a single XML file with repeated <entry> elements in the playlist (or repeated <playlist> elements in the tractor) would fix everything in one go.