mkvmerge: only warn for Matroska files with missing track UIDs

Before mkvmerge exited with an error. Even though such files violate the
Matroska specifications ignoring that violation is easy enough, a
warning will suffice.

Fixes #1050.
kax_analyzer: create track random UIDs if element is missing

Fixes the second issue mentioned in ticket #1050.
Translations: update list of translatables; update German