Allow marker-reference inside volume-transition

[bertfrees]

I think marker-reference could in theory be allowed anywhere, but let's start with volume-transition because that might be easier to implement. The use case is to insert the current chapter title at the start of the new volume. Kind of like a running header, but not at the beginning of the page (header) but the beginning of the volume (volume-transition).

[bertfrees]

This was implemented in mtmse#11.