Fix sector type definitions.

- Sector types weren't being cleared when loading new configuration, so
  they "bled" from one map editing session to the next.

- When editing a ZDoom Hexen or UDMF map derived from Doom, the Doom
  sector types were listed, even though those sector types are only
  recognized for Doom-format maps.

- Several ZDoom sector types were misnumbered.

- Strife didn't have its own sector types list, but it uses slightly
  different ones from Doom.
Restore Boom-derived sector type flags to ZDoom.

They were disabled when Boom features were set to only be enabled in
Doom-format maps, but unlike some other Boom extensions, these still
work.  They're now their own configuration property, which also allowed
for removing a lot of special-casing by map format.

The sector special propgrid panel now also shows the name of the sector
special.
Merge pull request #215 from eevee/restore-sector-specials

Fix sector special definitions and restore flags under ZDoom.