FGManager: Block VSFilter when XySubFilter will get used.

This is generally a temporary solution.

In the future the amount of things in the check might have to
be adapted depending on the implementation of MadVR's subtitle
interface.
MainFrm: Prevent XySubFilter from being removed from the graph when reading external subtitles.

Temporary fix, either offer external subtitles as a pin on the graph or remove CleanGraph() entirely.