introduce MetaTags api

In some games or players it would be possible to print a title and
some minor tags about playing music.

There are 5 added functions which can give meta-tags from music if
they are available or supported by a codec:
- Mix_GetMusicTitle() gives song title.
Unlike Mix_GetMusicTitleTag, returns filename if tag is blank.
- Mix_GetMusicTitleTag() gives song title
- Mix_GetMusicArtistTag() gives artist name
- Mix_GetMusicAlbumTag() gives album name
- Mix_GetMusicCopyrightTag() gives copyright tag

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