Add UDMF linedef arguments and make slope specials use them

This is the first part of UDMF that requires an internal binary-to-UDMF conversion (which is done in P_ConvertBinaryMap). Because this outright changes the linedef type after spawn, it could potentially break Lua scripts that scan the linedef type in some way. Therefore I'm keeping this out of next. Instead I made a new branch udmf-next, which is where all UDMF-related stuff will get merged into from now on. Once UDMF is complete enough to put in the next release, we can merge udmf-next into next.

Documentation of the current state of UDMF (including how the linedef specials etc are set up) goes here from now on: https://wiki.srb2.org/wiki/User:MascaraSnake/UDMF/Current

srb2win-linedef-args.exe (Updated 7/1/20)