Always show action special/thing type args, even if unset

Fixes #99.

Works for both linedefs and things, UDMF and Hexen.  Tried to consolidate all the code duplicated between lines and things, too.  My C++ is not fantastic, so apologies if this is actually all terrible.  :)

Also fixes a copy-paste bug where a ThingFlag's type was reported as `TYPE_LFLAG`, though I have no idea whether that actually affects anything.

One minor caveat: some of the arg definitions try to "erase" inherited args with e.g. `arg2 = "Arg2"`, and this code still thinks that's a defined argument, so it'll still show.  Data format may want extending to support that for real.