Some refactoring to allow further changes to support searching for skos concepts by the label (in the FindPanel)
Got the mechanics of the FindPanel working for SKOS terms, based on their prefLabel and altLabel

Needs some refactoring, but this will come later in a general
refactoring sweep
Fix to using the correct label when applying a skos term to a cell (previously was using the URI fragment)
Bug fix to Tree selection
Protect against a null when selecting the Thing (top class), or if a label cannot be determined for another reason
Made appearance of Top node for SKOS consistent with the rest of the tree