Changes related to external subtitle renderers

The commit messages are rather straightforward. The last thing was something requested by cyberbeing, and is the most controversial of them all. Discussion welcome. Also, may I say that I hate the Github pull request thing with a passion? It wouldn't even let me use my old repository as the base since it "isn't a fork" (originally used to house my git svn clone), making me fork and add a new remote for this  >:| .

tl;dr:
- Adds GUIDs of VSFilter and XySubFilter to moreuuids.h
- Uses the VSFilter define in AppSettings
- Blocks VSFilter in the currently only case where XySubFilter would get used.