AspectCopier must copy xtend/K3 code instead of the generated java code

[dvojtise]

In order to have only one project for the runtime language,
the AspectCopier copy only the generated java code from xtend/K3
(probably because this is easier to reuse java code manipulation library and also due to diverse-project/k3#45)

However, copying only the java code prevents K3 from being able to optimize setter/getter code and force the use of inefficient reflexive code (diverse-project/k3#68)

I suggest that the AspectCopier should copy and adapt the K3 code instead of the generated java code.