Unified LV2 core/extensions code documentation

I've reworked the LV2 documentation generation system to generate all code documentation for lv2plug.in in a single Doxygen run. This means the core and all extensions' documentation are hyperlinked between each other, and there are handy global indices of everything.

See http://lv2plug.in/ns/doc/html/ for an index.

As before, you can go directly to the URI of any lv2plug.in extension in your browser for documentation, which links to the code documentation for that particular extension.

Hopefully this reference proves useful for implementers.