gstdoc-scangobj: Add a mechanism for adding 'implicitly created' GTypes into the scan, allowing for documenting plugi...

Original commit message from CVS:
* gstdoc-scangobj:
Add a mechanism for adding 'implicitly created' GTypes into the
scan, allowing for documenting plugin-private base classes that
provide signals or properties for public elements.
* gtk-doc-plugins.mak:
Use $(builddir) instead of $(top_builddir) in a few places - there's
no need to hard code 'docs/plugins' as the only useable path.