This method displays the given URL in he appropriate viewer. The
appropriate viewer is user defined, according to these rules:
1. Extract the protocol from URL. This is defined as everything before
the first colon
2. Check if the key /desktop/gnome/url-handlers/protocol-show
exists in the gnome config database. If it does, use this as a command
template. If it doesn't, check for the key
/desktop/gnome/url-handlers/default-show, and if that doesn't exist fall
back on the compiled in default.
3. Substitute the %s in the template with the URL.
4.Call GnomeExecute.shell, with the expanded command as the second
argument.
- True if the url was shown. False if there was an error