Add mpls merger

[RyougiKukoc]

• Merge (append) mkvs after muxing (audio tracks must be FLAC?).
• Use core.mpls to read clip names from mpls files.
• Use mkvmerge to append tracks.
• Two options for audio: 1. Use special version of mkvmerge to merge (append) FLAC. 2. Re-encode.

[RyougiKukoc]

Interface:

• Path to PLAYLIST.
• Path to workspace (to search with glob, match the first one in list).
• Executable fp like other modules.
• Option to choose audio dealing mode.

[RyougiKukoc]

Question: what to do with audios that is larger or shorter than video duration?
Answer: need not to do anything, if the track duration has not changed, everything should be the same as playing m2ts streams, which is serially appended as specified in mpls files.

[RyougiKukoc]

Donot append flac: https://gitlab.com/mbunkus/mkvtoolnix/-/wikis/Appending-&-splitting-FLAC-audio-tracks-not-supported

[RyougiKukoc]

If more than one volumn of BDMV exists, sort the mkvs according to volumn titles first.

[RyougiKukoc]

This function is release at https://github.com/RyougiKukoc/rkstool/blob/main/rkstool/mpls_merger.py
Any test is welcome