Refactor Tags and ReadTags a little

Renamed ReadTagsFromFile module to ReadTags.

Moved the Tags data type, along with emptyTag and modifyTag, to
it's own module, Gramophone.Core.MediaController.Tags.
Clean up ReadTags.hs. No change in functionality.

ReadTags.hs was some of the first Haskell code I ever wrote. Now
that I know Haskell better, I can write a much cleaner version.
Still in progress.