Extend BaseLayerPicker to support terrain

1. Add Imagery and Terrain sections to BaseLayerPicker
2. Remove ESRI World Street Map, Stmen Watercolor, and MapQuest OSM from the default provider list, these didn't add anything extra and the number of items was getting out of hand once terrain was added (Watercolor was only half the world too).
3. Add WGS84, STK World Terrain, and smallTerrain to the default terrains.
4. I had to rename a ton of stuff to make properties like `selectedItem` imagery specific (it's now `selectedImagery`) and there's a new corresponding `selectedTerrain`.
5. The imagery section or terrain section disappears completely if no terrain or imagery view models are supplied.
6. `CentralBody.depthTestAgainTerrain` is automatically enabled if anything other than WGS84 is selected.

While this was a simple and straightforward change to make, it's a large breaking change for anyone using the `BaseLayerPicker` directly.  Thankfully I expect almost everyone is using it through `Viewer`, where the breaking changes are all hidden away.

@emackey As I mentioned to you, while I'm mostly happy with the new styling; feel free to tweak (or suggest tweaks) if you have ideas for improving it.

Hopefully I didn't leave anything out.