Protect private definitions by the GEANY_PRIVATE macro in headers

In addition, this patch defines the GEANY_PRIVATE macro for the tag
manager library where it wasn't defined before, removes 2 tag manager
headers from distribution as they are not needed by plugins and
in the tag manager changes the docstrings to ordinary comments for
private definitions.
Those need to be listed somewhere, otherwise they won't be part of the tarball.
We could list all headers in `_SOURCES` and indeed just add the ones that *have* to be installed in `_include_HEADERS`.