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.
Is there a reason (I can't see one) why this loop uses an index instead of simply being:
`for (TpeImageFeature tif : _featureList) { ... }`
?