myth protocol: add a MUSIC_TAG_ADDIMAGE command

This command will be used by the music metadata editor to add an image to a
tracks tag.

format: MUSIC_TAG_ADDIMAGE <hostname> <songid> <filename> <imagetype>
myth protocol: add a MUSIC_TAG_REMOVEIMAGE command

This command will be used by the music metadata editor to remove an image from
a tracks tag.

format: MUSIC_TAG_REMOVEIMAGE <hostname> <songid> <imageid>
Merge remote-tracking branch 'upstream/master'