Key management

[maRci002]

This PR adds some key management feature for this project.
Basicly keys preserve state when widgets move around in the widget tree.

There was two problems:

1. If I have forexample 3 TileLayer layers (I., II., III.) and if I swap II. and III. widget layers then new II. layer will have the old II. layer's state and new III. layer will have the old III. layer's state => so the loaded _tiles which are stored in state will have a hard time because it will show old tiles and panning will load new ones. If no keys provided then Observe TileLayerOptions changes #584 PR will think url has been changed so the wrong _tiles will we reloaded, however this can avoided by using keys. This may happen forinstance if you want swap two wms layers.
2. In flutter_map_state.dart the layerStack may move around in widget tree which causes for statefull layers (forinstance TileLayer) to fully loose their states, this happens if map is rotated and/or intercative property changes so GlobalKey needed.