Navigate through POIs and roads using VoiceOver

[1ec5]

MGLMapView currently cycles through the visible annotations when you use VoiceOver’s leftward and rightward swipe gestures. Once #352 lands, we should also have it navigate through POIs and roads, as MapKit does. See this comment for an example video of how MapKit implements road selection: #1496 (comment).

GL styles can be a lot more freeform than the map types MapKit provides. So while it may be possible to query for all visible labels or all visible point-placed symbols, it may not make sense to do so for all styles. On the other hand, we shouldn’t rely on developers to manually query the map and maintain their own accessible element structures – it’d be enough work that no one would make their maps accessible. Instead, we may want to expose APIs that make it super easy for developers: for example, an MGLMapViewDelegate method might return an array of layers to query for accessible features or a predicate to filter features by.

/cc @boundsj @friedbunny

[1ec5]

Implemented in #9950, which is about to ship in iOS SDK v3.7.0.