Initial checkin of Gtk rewrite

- Rewrite xdsc in Python/Gtk3
- Provide the Glade UI file for the new xdsc
- Provide an icon for xdsc (stolen from diswww)
Remove C version
Update man page
Tweak font and UI

- Per andersk's suggestion, Use Gio.Settings to determine the preferred
  monospace font, rather than hardcoding Courier.  However, we don't use
  get_string() since that segfaults if the string isn't there.  Fortunately,
  Gio.Settings pretends to be a dictionary.
- Add a margin in the transaction textview widget