Change group
Generate extension properties for getters and setters

It's all very messy currently but everything's at least working
after lots of trial and error. The next commit(s) will introduce reuse,
abstractions, refactorings and hopefully simplifications.

Note that I could have squashed this commit but I wanted to save
it in case the situation arises when I really do need to work
with allMethods instead of methods, however unlikely it may be.
Improve code generation

The code generation scripts for the kalium and xetter
extension properties now share a lot of code and are
simplified.