Fix a crash in the panel plugin and improve cli options handling.

Don't exit the gtk main loop when running the panel plugin, or the
panel plugin will be killed.

Don't accept two region options or two action options. Warn the user
that mouse, delay and action option are ignored when passed without a
region option. At the moment, those error and warning messages are only
displayed in logs or in a terminal. I don't think it's worth showing an
error dialog for this.