Cleanup
Add methods for returning all layer ids, layers and also for the layerId : layer dict
Make the mapLayers object return map layers by id
Don't work with wrapped mal layer objects returned by wrapped objects directly as that seems to be broken at the moment in PySide
Return map id and label directly

As returning a layer wrapper object from the group wrapper seems to stop working (looks like the object in NestedWrapper gets corrupted somehow) after a while,
we need to return the values directly.