Fix warnings

[mtschoen-unity]

Purpose of this PR

Fix warnings due to 2018.3 upgrade and default runtime becoming .NET 4.x

Testing status

Tests pass locally. I no longer see EditorXR-related warnings when recompiling in-editor or making player builds. Did a basic smoke test in editor, play mode, and player build.

Technical risk

Very low--mostly pre-processor defines and prefab API updates

Comments to reviewers

That first commit is pretty hefty, so let me break down the changes:

• The new prefab workflow requires us to change PrefabUtility.GetPrefabType to PrefabUtility.GetPrefabAssetType, PrefabUtility.CreatePrefab to PrefabUtility.SavePrefabAsset, PrefabUtility.FindPrefabRoot to PrefabUtility.GetOutermostPrefabInstanceRoot, and PrefabUtility.GetPrefabType to PrefabUtility.GetPrefabInstanceStatus
• The process in AssetGridItem where we reconnect the preview object to its prefab is no longer viable. Instead, we must re-instantiate the preview with PrefabUtility.InstantiatePrefab. It is not possible to clone a non-prefab object whose children are prefabs and maintain prefab links.
• There were many places where private SerializeFields have no setters. This results in a CS0649 warning, which was suppressed in the legacy runtime. For now, I have used #pragma warning disable to suppress the warning in the new 4.x runtime. I could also have fixed this by adding public setters, but they would have been unused, and in many cases would have added a lot of noise to our classes.
• The fixes for player builds came down to more aggressively adding #if UNITY_EDITOR, and working around code that should be editor-only.
• As I went along, I addressed spelling mistakes, unused fields, and field order issues that I came across.
• In doing a final audit of these warning directives, I found a few places where we had mismatched disable/restore directives, and fixed them.