Make sure canShowCallout properly reflects whether the annotation can actually display a callout or not (regardless of whether it's a default callout view or not)

Signed-off-by: Blake Skinner <bskinner@mit.edu>
The 'enabled' property toggles the annotation's visibility, not whether it can show a callout or not. Use the canShowCallout property here instead.

Signed-off-by: Blake Skinner <bskinner@mit.edu>