Make -[MGLMapView showAttribution:] public

[1ec5]

-[MGLMapView showAttribution:] should be a public IBAction. That way the developer can implement their own attribution button and easily hook it up to the map view's attribution sheet. The current workaround requires setting the custom button's action to the built-in attributionButton's action, but that's fairly undiscoverable.

On macOS, MGLMapView has a public giveFeedback: action for the same purpose. (Copyright notices are displayed on the map instead of on demand, hence the naming difference.)

/ref #1781 (comment)
/cc @frederoni @nosyjoe

[1ec5]

This is essentially a slightly more flexible analogue of mapbox/mapbox-gl-js#1485.