working on #62: adding missing implementation for ModeLayer to the method getKeyforLayerElement
working on #62: added a forceMode method to ensure to refresh the PNode and added a dummy PNOde to the layer in putModeLayer, to avoid NPEs when the MappingComponent is not yet listening to the layer
working on #62: add the model reference to the layer (if applicable) before the layer is added to the model