[MapModel]: Add "highlightLayer" concept

[mbeckem]

Temporary graphics layers such as the measurement vector layer or the geolocation highlight use a custom layer and add it to the OpenLayers map via olMap.addLayer.

Care must currently be taken here to make sure that the new layer is really added on top of all other operation layers (timing might be a problem here if the map model is still initializing). The current solution is to set the z-index to a very high value; however this should only be used internally and may be difficult to maintain in the future.

A better solution would add a new interface to the map model: mapModel.addHighlightLayer that automatically adds an (anonymous) layer to the top of all operational layers.

Edit: because we are using z-index for our operational layers, all anonymous layer must have a z index as well (the default z-index is 0 which would result in them being at the bottom)