Render function candidates without using Sphinx directives

[jakobandersen]

The doxygenfunction directive works by

1. finding all functions in the XML which matches the given name (without potential parameters)
2. rendered each to docutil first nodes, and then converted the nodes to a string. The docutil nodes are discarded.
3. the string is then parsed and an attempt at matching the parameters with the given parameters is made (this is where What is the best way to resolve the Warning re: multiple matches for a function? #289 is relevant).

Step 2 was performed using the normal rendering machinery, meaning Sphinx is told about the declarations, without them being present in the output. This results is "Duplicate declaration" warnings.

This PR uses the DefinitionParser from the C++ domain to directly parse the function signature without using the domain directives.

Fixes #504 and perhaps also #458.

(The change of base class for the BaseDirective to SphinxDirective is to get convenient access to location and config data.)