have polygon and linear ring icons rendered by PlacemarkLayer

* makes the icons clickable, i.e. a placemark dialg can be shown

Partly re-applies b1a507ba
(leaves out the building label changes unrelated to the comment above)

Still needs a bit of tweaking to avoid too much label clutter.
However with rings and polygons setting their zoomLevel to 17 this was
reduced to the highest levels in the meantime. Fixes icon render order.