Don't show windows without icons in the task list

[joewing]

Reported via email, my interpretation follows.

I'm not sure if this is in general a good idea, but when a window does not provide an icon, maybe that should be a hint to JWM to not show it in the task list. A group option with nolist is likely possible, but it probably shouldn't be necessary (and maybe difficult to accomplish depending on how WM_CLASS is set).