SelectionListener is called twice on selection

[Christof Dallermassl]

The DataSet SelectionChangedListener.selectionChanged is invoked twice when the user selects a node: first the selection is cleared (and the SelectionChangedListener informed) and then the selected node is added and the SelectionChangedListener is informed.

Problem is in DataSet:

	public void setSelected(Collection<? extends OsmPrimitive> selection) {
		clearSelection(); // here the listener is informed
		for (OsmPrimitive osm : selection)
			osm.selected = true;
		fireSelectionChanged(selection); // here the listener is informed
	}