Change the add child piece into a treema-row

[sderickson]

Right now the DOM element for adding an item to an array or a property to an object receives a unique focus. Instead, give it a Treema node row so that it can be selected like any full fledged Treema row. Not sure how best to share that logic. Possibly have the add property/item row be a subclass to a common superclass with the Treema node. When it's done, update the navigation systems (pressing tab, enter, or arrow keys) to make sure they still work.