This change removes most of the ways you used to be able to apply a style to the map. Building on #1163, `styleURL` (HTTP(S), mapbox:, asset:) is the canonical way to apply a style, and `mapID` is a convenient shorthand for Mapbox-hosted styles. A relative style URL is interpreted as a path relative to the app’s main bundle. We now construct asset: URLs in lieu of “bundled style names”.
`Map` should be able to deal with having no access token or JSON even while it’s running. Most of `Map` accounts for this situation, but `reloadStyle()` incorrectly assumes that one or the other is set. This change corrects the assumption in `reloadStyle()`.
Also expose the access token and style name to clients as ordinary KVO-compliant properties. Actually, they’re not so ordinary because they’re inspectable!
Ref #1070, #1147
