MetaIOAVFComment: don't call av_estimate_timings() in getTrackLength()

It's not needed with current ffmpeg and was causing a segfault while scanning
for music files.

(cherry picked from commit c2128934644892df613590dda7e6017b72c14f94)
MetaIO: validate the files extension before creating a tagger

This checks the file extension is one we know how to handle before
creating the tagger. Return NULL if not valid.

This fixes a bug with the music file scanner trying to add files with an
unknown file extension.

NOTE: bumps the ABI version so compile and install myth core before compiling
the plugins

(cherry picked from commit 47217ca10adae1e03a37b396d64b605c8f18e6b0)
Bump the ABI version after f359a4ebd3c
MythMusic: make sure we null check the returned tagger after 47217ca10a

Also make sure the tagger is deleted after use.

(cherry picked from commit b383c329417a4e4b0b786e51e0d24962728a738b)
MythMusic: avoid storing a pointer to the current playlist in the music player

Always get the current playlist from MusicData to avoid possibly referencing a
stale pointer. This can occur while doing a rescan.

Also add NULL checks everywhere just in case.

Refs #11906.

(cherry picked from commit 193dc636e5fa5ac5edd3589094a58e79a6ab779b)
MythMusic: Fix detection of compilations using special MusicBrainz Artist ID

Look at all the fields in the 'TXXX=MusicBrainz Album Artist Id' frame for the special
various artists ID not just the first one which in current version of the taglib
contains the tags description.

Refs #10235.
Fixes #12000.

(cherry picked from commit 6530ac633300a32c76db0d10cf8d107aa712a62c)
Merge branch 'fixes/0.27' into stable/0.27