#9 log error when selectedNodes contains ws twice

define a watch for the selectedNodes property and check if it contains
the same worldstate multiple times. If so log an error to the console to
propagate this deficiency
fixed #9 demo app only provides root nodes

the demo application should only provide the top level root worldstates
to the tree widget. There is no need to provide a set of selectedNodes.
Merge branch 'bug/9-multiple-selected-nodes' into dev