* src/aboutdialog.cc (package_title): New string constant which
defines the package title string plus surrounding markup.
(AboutDialog::create): Use the separated package_title constant.

* src/aboutdialog.{cc,h} (AboutDialog::create): Change the return
type to std::auto_ptr<Gtk::Dialog> because copying-destroys-source
is just what we want.   Calling release() to achieve exactly that
was a silly idea to begin with.