Reintroduce the CtModifiableReference interface that existed in previous versions of Spoon.

This pull request reintrodruces the CtModifiableReference interface that used to exist in previous versions of Spoon, e.g. see 1.4.2. The implementation of this interface is still there, hence the fact that this pull request does not contain any code, just an interface declaration, and the fact that the code yet still compiles.