AppSettings/FGManager: Block VSFilter when xySubFilter will get used

FGManager now uses AppSettings.h, as there are now defines for
VSFilter's and xySubFilter's GUIDs. Also a new helper function
IsXySubFilterInstalled() has been added, and used from FGManager.

There was an idea about disabling xySubFilter when the ISR is
usable and enabled, but since the renderer is supposed to load it,
setting a "do not use" merit in FGManager will not have any effect
on the result.