Revise metadata output

[corybarr]

Right now, metadata is exported as key/value pairs. Many of the values are references to other items. They get written out as the element ID, which is currently an integer saved as a string. For example, here is a standard window:

This tool in the screenshot drills into the references such as "Category" to display the name. The glTF exporter will export the elementId of the value: "category": "-2000014"