Don't offer number of threads option if wrapper does not support parallelism

[mhucka]

Sarah pointed out that in the Preferences panel, the option for overriding the number of threads at the bottom of the panel should be grayed out if the currently-selected wrapper does not allow multiple threads in the first place.

This is going to be a bit awkward to implement because of the separate objects used to implement the wrapper panel and the rest of the preferences panel.