[libxml++] dtdvalidator patch integration

[Christophe de Vienne <cdevienne alphacent com>]

Hi,

I've just commited the dtdvalidator patch proposed a few month ago by 
Guillaume Arreckx.
I changed few things from the original patche :
- renamed *.cpp -> *.cc
- fixed a few comments which where copy/paste from other files
- replaced std::string with Glib::ustring
- Added Dtd::cobj, since the patch rely on it.
- added a validaty_error as suggested by jon

I put the inputbuffer classes in the io subdirectory, and created a new 
directory, "validators", to put the validator implementation.
I added an example in examples/dtdvalidation.

Feel free to comment, I'm sure there is some stuffs to improve.

I'll do a release of libxml++-2.7.0 when it will be roughly tested from 
cvs and the htmlparser patched reviewed and eventually included.

Regards,

Christophe

PS: Guillaume, you should subscribe to the mailing if you want to 
discuss with the rest of the team.