Be more consistent with regard to the RootIndexDisplayHint in ObserverWidget trees.

[JPNaude]

At present, the activeHints() is used to determine the RootIndexDisplayHint in both ObserverWidget and ObserverTreeModel. This is not optimal, it should only use the top level observer's hints to determine the RootIndexDisplayHint. It won't be an issue in most cases, but if a tree contains nodes with different RootIndexDisplayHint hints and the ObserverWidget is set to use the observer's hints, we might see funny things happening when selection changes between nodes with this hint set differently.