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.
Add support for bitflags and enums in the arg editing panel.

Support was already mostly present in the configuration file format and
arg_t structure, but unused.  I used it for a couple common action
specials and updated the dialog to show custom controls when it can --
which should also get rid of the need for very long arg descriptions.

The line info overlay also now shows textual descriptions of these
argument types.

The property grid is not yet aware of any of this; it could easily be
made to support enum-style args, but there's no obvious way to cram
flags into such a small interface.
Fix some validator and layout issues with ArgsPanel.
More action panel UI: rewrap desc, make it scrollable if necessary.
Define flags/choices for a handful of arbitrary ZDoom specials.
Merge pull request #217 from eevee/action-special-arg-types

UI and definitions for more specific action special argument types
Merge branch 'master' of https://github.com/sirjuddington/SLADE into osx