MGLCalloutView should extend UIView, not NSObject

[1ec5]

The MGLCalloutView protocol extends NSObject, but the documentation for it says an instance of MGLCalloutView must be a kind of UIView, and in fact that’s the assumption this line makes. We should correct the declaration of this protocol to extend UIView as documented.

/cc @frederoni

[1ec5]

My bad: a protocol can only extend another protocol, not a class. It turns out that we never say id <MGLCalloutView>, only UIView <MGLCalloutView>. As long as we’re consistent about that, there’s no potential for incorrect behavior.