matroska: K&R formatting cosmetics
matroska: Factor out write_track from mkv_write_tracks
matroska: Factor out mkv_write_stereo_mode
matroska: Factor out mkv_write_cue_entry
matroska: Move codec-specific functions in a stand-alone file
matroska: Provide an option to set the version explicitly

StereoMode is a version 3 element, CodecDelay is a version 4 element

Omit them if a noncompatibile version is requested.

Do not set the doctype version to a value higher by 2 by default to
support widespread implementation.