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)