Myth Protocol: add a MUSIC_TAG_UPDATE_METADATA command

This command will be used to update the metadata in a music tracks tag.
It assumes the new metadata has already been saved to the database.

format:-
MUSIC_TAG_UPDATE_METADATA <hostname> <songid>
MythMusic: don't send SCAN_MUSIC and MUSIC_TAG_UPDATE_VOLATILE in the UI thread
MythMusic: fix saving the metadata to the tag on the editmetadata screen
Fix install paths for libdvdnav/read include files.
Initialize memory used in Jitterometer::GetCPUStat to keep valgrind quiet and happy.
Invalidate the cache after deleting a video file to ensure the free'd entry is removed.

Fixes #12052
Merge remote-tracking branch 'upstream/master'