WIP: Gtkmm about box

Open username-removed-1411004 requested to merge gtkmm-about-box into master

This branch rebases our "About" dialog on the "standard" Gtk::AboutDialog class, which didn't exist when our dialog was first made. The main advantages are:

  • Much better consistency with appearance & behaviour of other Gtk+ applications
  • Automatic parsing of email addresses & URLs in the "credits" section
  • Much cleaner and more maintainable code

The main changes vs master are:

  • Rebased on Gtk::AboutDialog (see above)
  • Install AUTHORS and TRANSLATORS files to share/inkscape/doc
  • Read credits from AUTHORS & TRANSLATORS files rather than needing to keep a hardcoded copy up-to-date
Edited by username-removed-1411004