Simplify and make more efficient building Python docstrings.

Make the rules for combining explicitly specified docstring, autodoc one and
the one obtained by translating Doxygen comments implicit in the structure of
the code itself instead of writing complicated conditions checking them.

This results in small changes to the whitespace in the generated Python code
when using autodoc, but this makes it PEP 8-compliant, so it is the right
thing to do anyhow.

Also cache the docstring built from translated Doxygen comments. The existing
code seemed to intend to do it, but didn't, really. This helps with
performance generally speaking (-10% for a relatively big library using a lot
of Doxygen comments) and also makes debugging Doxygen translation code less
painful as it's executed only once instead of twice for each comment.

Finally, avoid putting "r", used for Python raw strings, into docstrings in C
code, it is really not needed there.
Remove long line wrapping from Python parameter list generation code.

This doesn't play well with PEP8 checks which imposes very strict continuation
line indentation rules which need to be _visually_ aligned, i.e. the subsequent
lines must be indented by the position of the opening bracket in the function
declaration line, but the code generating the parameter lists doesn't have
this information and so it's impossible to do it while avoiding either E128 or
E123 ("continuation line {under,over}-indented for visual indent" respectively)
error from pep8.

Moreover, the wrapping code didn't work correctly anyhow as it only took into
account the length of the parameter list itself and not the total line length,
which should include the function name as well.

So just disable wrapping entirely, long lines shouldn't be a problem anyhow in
auto-generated code.
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.
Also merge Doxygen comments for overloaded constructors in Python.

This was done for plain functions/methods but not constructors, which don't
have "kind=function" attribute.

Just don't check for "kind" at all, the presence of "sym:overloaded" should be
good enough to indicate that it's an overloaded something.

Also add a test for overloaded constructors documentation.
Fix translated Doxygen comments for overloaded functions in Python.

The most important change is to ensure that the "Overload" headers added by
PyDocConverter code are indented correctly, i.e. using the same indent level
as the comment itself, otherwise Sphinx directives such as :param: were not
parsed correctly.

Also add a vertical separator using reST "|" formatting character to increase
separation between the overloads.
Document the return type when translating Doxygen @return to Python.

In addition to translating the comment itself, also document the type of the
object returned. This is consistent with generating not only :param: but also
:type: for the parameters documented using @param.