Load themes from backend and configures background layers if needed
Preload all WFS DescribeFeatureType In order to limit the network overload, the servers calls are done sequetially and each call will wait the previous one to be done
Will create layer and child layers if elem passed is a group of layers
either a layer or a group of layers
in case children are not mixed layers, the parentServer will apply for all children
the order in the layer list
the created girafe layer
Static