Deprecate 'selected' key in obj dictionary

Now that selection is switched to a single var in ResourceRoot containing the selected obj, I should be able to fully deprecate the 'selected' key in the obj dictionary, and all code references to that key.

I need to double check this is the case, then clean up the code.

Note that something like the selected key would need to come back to support multiple selection in the treeview, but I don't see a need for that.