Add documentation for 3D Tiles integration into Cesium for Unreal

[kring]

Fixes #1542

It basically explains how the Cesium Native integration points for 3D Tiles rendering are implemented in Unreal Engine.

As always, there's probably a lot more that can be said here, but I think this hits the most important points.

This PR also has some light refactoring, because sometimes it's easier to fix weird things than to describe them:

• Moved UnrealResourcePreparer out of Cesium3DTileset.cpp and into its own source files. Also renamed it to UnrealPrepareRendererResources to match the interface name.
• Renamed LoadModelResult and other similar types to LoadedModelResult. This is to match LoadedTextureResult, which is the only one among this family that is public.
• Separated FCesiumUseExistingTextureResource into two classes, the other called FCesiumPreCreatedRHITextureResource. Previously this class had two constructors that were used in two entirely different ways, for no really good reason.

[azrogers]

This link fails to resolve because it's not named UCesiumEncodedMetadataComponent anymore - it's UDEPRECATED_CesiumEncodedMetadataComponent. I believe UCesiumFeaturesMetadataComponent is what we're using now instead, but @j9liu would know better than me.

[azrogers]

Besides the one comment, it's looking good. I made a couple of copy edits but nothing major. Feel free to merge this once that comment is resolved!

[kring]

Thanks @azrogers! It turned out UCesiumFeaturesMetadataComponent was missing from the docs, too, but for a different reason. Apparently constructs like this confuse the Doxygen parser and so any class decorated this way won't show up in the docs at all:

UCLASS(ClassGroup = (Cesium), meta = (BlueprintSpawnableComponent))

However, this works fine:

UCLASS(ClassGroup = Cesium, meta = (BlueprintSpawnableComponent))

So I changed a bunch of occurrences of this, and now a handful of types that were missing are showing up. Not sure if there's any way to make Doxygen report problems like this, rather than silently excluding the type. I couldn't find a way with a brief look.