MapboxMap should have methods to list layers and sources

[1ec5]

MapboxMap should have the following methods, which are possible now that #6097 has landed in master:

• List<Layer> getLayers()
• void addLayerAfter(Layer, String) (not sure about the naming, but adding a layer above a particular layer is more commonplace than adding one below it)
• void addLayer(int, Layer)
• Layer removeLayer(int)
• List<Source> getSources()

Additionally, removeLayer(Layer) and removeLayer(String) should return Layer.

These methods will all be present in iOS SDK v3.4.0 and macOS SDK v0.3.0. Labeling as GL JS parity because these operations are also possible in GL JS using getStyle() and setStyle().

/cc @ivovandongen

[tobrun]

Capturing from talks with @ivovandongen that we should hold on implementing this until #6533 lands else this would result in doing the work twice.

[deinlandel]

Voting on this. Currently I'm trying to change map locale.

I'm trying to do this according to https://www.mapbox.com/help/change-language/
But "city_label_big" layer always returns null.
As well as layers

country_label
marine_label
state_label
place_label
water_label
waterway_label
motorway_junction
airport_label
rail_station_label
mountain_peak_label
road_label
poi_label

I tried every possible layer name I encountered in various mapbox default styles, and it always return null. Without the ability to list layers I can't even understand what's wrong with my layer names.