matroskamux: Add possibility to set DateUTC with a DateTime tag event

Submitted by Linus Svensson

Assigned to Tim Müller @tpm

Created attachment 372590
Use nanosecond precision for DateUTC

Add support to set DateUTC with a DateTime tag event. If no event is sent to matroskamux, the current behavior remains.

I need the nanosecond precision for my use case. Any objections on my approach to always use that precision?