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.
Only tm_source_file, tm_work_object, tm_workspace are defined as INPUT in Doxyfile, so we should probably avoid removing doc-comments for other files IMO.