Replace MapChange enum and Backend::notifyMapChange with dedicated observer interface

[jfirebaugh]

The MapChange enumeration values have their roots in MapKit semantics -- yet are increasingly deviating from those semantics, or expanding on them. We should replace them with an API that is less influenced by the API of a particular platform, and more influenced by the pattern we use elsewhere for observation, e.g. the style observer. It should be general enough that MapKit-like change notifications can be built on top of it.

[1ec5]

The MapChange enum was never particularly well-suited for MapKit semantics, either, because it was impossible to provide relevant context, such as a camera or error pointer, with a change notification. Awhile back, I started experimenting with making map change notifications more object oriented in the 1ec5-delegate-fail-loading-6142 branch, but imitating the style observer would be even better.

[jfirebaugh]

Related: mapbox/mapbox-gl-js#3244