Refactor: move makeParameterName() to common Language base class.

This method was duplicated more or less identically for 4 languages and will
be needed for another one soon, so put it in the base class from which it can
be simply reused instead.

No changes in the program behaviour whatsoever.
No real changes, just make PYTHON::check_kwargs() const.

This will allow calling it from const methods too.
Don't always use "*args" for all Python wrapper functions.

Due to what seems like a bug introduced during Python 3 support merge, all the
generated Python functions used the general "*args" signature instead of using
the named parameters when possible.

This happened due to is_primitive_defaultargs() always returning false for the
functions without any default arguments as "value" passed to convertValue()
was NULL in this case and convertValue() always returns false for NULL.

Fix this by checking for value being non-NULL before calling convertValue().

Doing this exposed several problems with the handling of unnamed, duplicate
(happens for parameters called INOUT, for example) or clashing with keywords
parameter names, so the code dealing with them had to be fixed too. Basically
just use makeParameterName() consistently everywhere.
Allow using enum elements as default values for Python functions.

Enum values are just (integer) constants in Python and so can be used as the
function default values just as well as literal numbers, account for this when
checking whether function parameters can be represented in Python.

Also rename is_primitive_defaultargs() to is_representable_as_pyargs() to
describe better what this function does.