[sigc] git repo renamed to libsigcplusplus

[Murray Cumming <murrayc murrayc com>]

I have renamed the git repository from libxml++2 to libsigcplusplus. I
have already updated the website (whose source is in docs/website in
the repo) and jhbuild.

The 2 in libsigc++2 was left over from the svn to git conversion a few
years ago, and it would be confusing if we do a libsigc++-3.0. Also,
the ++ seems to have broken mirroring on github. That should work again
soon, and maybe we could even accept pull requests that way too.

-- 
Murray Cumming
murrayc murrayc com
www.murrayc.com