[overlay] reset implementation and simplify with lovely preprocessor macros

[myk002]

• removes all logic from overlay and leaves only a plugin skeleton that interposes all the viewscreens
• adds framework for preprocessor macro-based list transformations:
  • MAP applies a function macro to a list of arguments
  • MAP_JOIN applies a function macro to a list of arguments and inserts a given separator between the results
  • MAP_LIST a specialization of MAP_JOIN with a comma as the separator

does the absorbed code need a LICENSE entry?

[myk002]

It appears that windows does not like the preprocessor macros. I'll have to research that a bit to see if there's something I can do for compatibility.

OTOH, we're about to update the windows compiler for the steam version, right? That might fix the problem too.

[myk002]

This appears to be a known issue. Fixed in VS 2019 with /Zc:preprocessor and in VS 2017 15.8 with /experimental:preprocessor (see here). We could enable the latter compilation option with our current build, I believe.

[myk002]

it appears that our MSVC toolchain is too old even for the experimental flag. I'll revert the macros for now and revisit once we've updated our toolchain for the steam release.