Add a CMake function to add a kcfg file without kcfgc

kcfgc files are basically a list of arguments for how to genenerate the config class. Rather than having to create a kcfgc file manually every time, we can have CMake take care of generating the kcfgc file from arguments passed to a function. This removes a step of indirection that always felt awkward to me.

Right now it just allows all possible values from the kcfgc files, though a few where that are "true/false/list of values" are currently just boolean flags. We may want to consider how many of these are still actually useful though.