Implement override docstring controls

[lazka]

• In case the functionality of the overidden function differs from the original one it should be possible to replace the gir documentation all together.
• In case the override just adds some additional arguments or default values it should be possible to include the gir docs in the override docstring (something like "%INCLUDE%" which gets replaced)
• In case the override uses "*args, **kwargs" because of backwards compatibility or because argument processing is delegated to some helper function it should be possible to define a Python signature which replaces the real one as seen by Sphinx.

[lazka]

Simon suggested %gi_doc_string_body% instead of %INCLUDE%