Gk browser navigation workaround

Changed navigation of arrays to show like this:

Company
Person
    ["Alexander" ...]
    friends <Person>

(https://app.asana.com/0/15066612015532/16375424745326)

In the process i factored out property value descriptions and tooltip descriptions to a separate class, RLMDescriptions, which also takes care of object descriptions in the sidebar.

@alazier @astigsen @jpsim @yoshyosh 
OK, I added this computed property:

    - (NSMutableArray *)displayedItems
    {
        return displaysQuery ? displayedArrays : displayedObjects;
    }