4.9.1 (unstable):

* Change ABI name from libxml++-4.0 to libxml++-5.0
  There is also a libxml++-4-0 branch for the libxml++-4.0 ABI,
  which still uses glibmm-2.64.
  And there is a libxml++-3-0 branch for the libxml++-3.0 ABI,
  which still uses glibmm-2.4.
* Build: Remove dependency on glibmm and don't use Glib::ustring
  For now, the API uses an xmlpp::ustring alias of std::string
  where it previously used Glib::ustring.