Open magnet:, irc:, apt: and friends with an application

As of git 2011-03-05 02:40:00 UTC and Midori 0.3.3 you can:
1. Add a line to ~/.local/share/applications/mimeapps.list:

 x-scheme-handler/magnet=transmission-gtk.desktop

2. Install an application which advertises the scheme like so:

 MimeType=x-scheme-handler/magnet;

Note that incomplete .desktop files will silently fail and it will look as if it doesn't exist.