REL-2101: TPE strehl feature appears when it shouldn't

The crux of the change is in `TpeFeatureManager.setAvailableOptions` where we would leave the strehl feature (and others) visible in the `TpeImageWidget` despite removing their controlling checkbox from the toolbar.   Now the availability of a feature will impact whether it is known to the `TpeImageWidget` as well.  In addition I changed the name of the "visible" property to "available".  It was confusing to me because visible could be `true` and yet nevertheless the feature might not be displayed in the image widget.  This property just means available for selection/viewing in the image widget so the name "available" seemed more appropriate.
Probably this was written before Java had that feature.  There are parts of the OT that go back to the 1990s.  Thanks for pointing this out though. I didn't pay attention when I went through marking things `final`.  I'll update it now.