GEOT-4945: MapContent#addLayers throws NullPointerException if MapLayerTable enabled

Since the Layer MapLayerListEvent#getElement return is null when multiple
layers are added at once (with MapContent#addLayers) we instead rely on the
layer indices in the event to determine the layers that are to be added to
the MapLayerTable.
Merge pull request #606 from leif81/GEOT-4945

GEOT-4945: MapContent#addLayers throws NPE if MapLayerTable enabled