Node Explorer: Maintain references to nodes in the TreeView

[tylersmalley]

We want to improve the efficiencies for when we need to trigger a refresh on the Tree View. When creating a file or directory, we need to call this._onDidChangeTreeData.fire and pass the parent FileExplorer. However, we don't currently have a way to get this. Additionally, sometimes (as with drag and drop) we only have a Uri. We want to be able to lookup a FileExplorer instance by a Uri.