[Map Editor] Feature Request: MBF21 Support

[XaserAcheron]

Quick-ish feature request that came up in passing: SLADE's map editor doesn't yet have a config for the MBF21 standard (UDB's got one now, fof reference) -- tl;dr it's basically just Boom with a few new specials added; most of the new features in the standard are DEHACKED stuff you won't have to worry about. :P