changing terminal emulator

[Francis Ferrell <codepanda gmail com>]

I'm using kupfer v206 on gentoo under openbox (thanks for making a
launcher that doesn't depend on so much, like its alternatives).  I
use xfce's terminal and so don't have gnome-terminal installed at all.
 As a result, when I try to use the System Services plugin, it fails
because gnome-terminal isn't available for launching gksu.

When I look under the General tab in Preferences, I see that the
Terminal Emulator select box has no value and is grayed out.

How could I get xfce-terminal as an option in that drop down?
-OR-
How could I configure kupfer via ~/.config/kupfer/kupfer.cfg to use
xfce-terminal?

Gratuitous stacktrace:
> Error [kupfer.desktop_launch]: Error Launching  ['gnome-terminal', '-x', 'gksu', '/etc/init.d/lighttpd', 'start'] Failed to execute child process "gnome-terminal" (No such file or directory)
> Traceback (most recent call last):
>   File "/usr/share/kupfer/kupfer/ui/browser.py", line 1563, in _activate
>     self.data_controller.activate(ui_ctx=self._make_gui_ctx())
>   File "/usr/share/kupfer/kupfer/core/data.py", line 798, in activate
>     res, ret = ctx.run(leaf, action, sobject, ui_ctx=ui_ctx)
>   File "/usr/share/kupfer/kupfer/core/commandexec.py", line 303, in run
>     ret = activate_action(execution_token, obj, action, iobj)
>   File "/usr/share/kupfer/kupfer/core/commandexec.py", line 80, in activate_action
>     return _activate_action_single(obj, action, iobj, kwargs)
>   File "/usr/share/kupfer/kupfer/core/commandexec.py", line 88, in _activate_action_single
>     ret = action.activate(obj, **kwargs)
>   File "/usr/share/kupfer/kupfer/plugin/services.py", line 69, in activate
>     utils.spawn_in_terminal([sudo_cmd, leaf.object, self._command])
>   File "/usr/share/kupfer/kupfer/utils.py", line 203, in spawn_in_terminal
>     desktop_launch.spawn_app_id(term["desktopid"], _argv , workdir, notify)
>   File "/usr/share/kupfer/kupfer/desktop_launch.py", line 353, in spawn_app_id
>     return spawn_app(app_info, argv, [], workdir, startup_notify, screen=screen)
>   File "/usr/share/kupfer/kupfer/desktop_launch.py", line 404, in spawn_app
>     raise SpawnError(unicode(exc))
> kupfer.desktop_launch.SpawnError: Failed to execute child process "gnome-terminal" (No such file or directory)