Some enhancements and a const pointer type

This pull request contains three changes:
 1. I saw the earlier, the parameter of getParameterOfType has been made const, which is a good idea, but hasn't been done for the C interface as well
 2. I also noticed, that strings has been changed to const, I also chaged the type name in the code to const char *
 3. I introduced a new type, which is a const pointer (const void*). This is useful, so we can mock const and non-const parameters as well. Earlier, we were not able to mock const pointers, because the parameter was non-const. The parameter could be changed to const, but this way we are not able to return non-const pointers in mocks. So I introduced a separate type for const, but I am open for other solutions as well.