CustomOptions, Settings, and ProjectTypeVariables are all duplicate functionality

[alcexhim]

They are used for different purposes but they are all essentially duplicates of the same underlying functionality, i.e. instructing the UI to present different ways of modifying settings or options or variables. We should work on a single unified solution to present these settings and obsolete the repetitive code in the other areas. CustomOptions seems to be the most likely candidate for the most stable code.