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.
Fix doxygen warnings

Make documents_array public and accessible by doxygen and describe all
parameters of tm_source_file_new().
Define the GEANY_PRIVATE macro for tag manager in makefile.win32
Add all headers to libtagmanager_a_SOURCES in tag manager makefile

So they become part of tarball.
Merge pull request #351 from techee/guards

Protect private definitions by the GEANY_PRIVATE macro in headers