Prompt for access token in iosapp

[1ec5]

If no access token is found in user defaults at launch, iosapp prompts the user for an access token. Renamed the user defaults key for the access token to conform to Cocoa naming conventions. The MAPBOX_ACCESS_TOKEN environment variable is still honored.

Added an action to MGLMapView that reloads the current style. Note that it doesn’t clear the cache; it’s pretty much only useful for ensuring that a change in access token is reflected on the map or for forcing the map view to retry loading tiles independently of the exponential back-off strategy.

Fixes #3102 and fixes #4696.

/cc @friedbunny @kkaefer

[friedbunny]

As you noted, users will expect a -reloadStyle: method to fetch the current style from the network and display changes. Until we do have such an implementation, can we keep this as a private method or as a category/extension where necessary?

If we make this public now, let’s add a warning to the inline docs stating that it does not affect cached resources.

[1ec5]

I’d rather that iosapp rely solely on public API, and it does fetch the current style when the original requests failed due to network conditions. So let’s document the caveats.