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
Change default path for UI and icon file

The package is not debathena-specific, so we should use our own
directory in /usr/share
Clean up comments in get_meeting()

Explain why we do the check_update() hack, and remove incorrect
comments in docstring about caching connections.
Don't call invalid methods

Fix some leftover code from the DiscussWrapper rewrite.
Use new RCFile.delete() method

Use RCFile.delete() instead of writing our own.