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.