Add a way to get a specific marker

[TesteurManiak]

StatefulMapController has a markers property to return all markers but it has no way of getting a specific Marker or List<Marker> by using a name like this:

List<String> poiNames = ["poi1", "poi2", "poi3"];

List<Marker> poiMarkers = statefulMapController.getMarkers(poiNames);
Marker firstPoiMarker = statefulMapController.getMarker(poiNames.first);

It would help a lot to separate markers into multiple map layer.