New prefabs workflow support for patcher

[dmitry1100]

The patcher missed feature to edit patches inside prefabs. Since Unity 2018.3 there is a new prefabs workflow that loads prefabs into isolated preview scene which operates almost as play scene. This pull request extends patcher to edit prefabs based on the new workflow. The feuture is conditioned under UNITY_2018_3_OR_NEWER.

[dmitry1100]

I've force-pushed my feature branch with the following update: When the patch is part of prefab instance it is not enough to just Undo.RecordObject() to 100% garanty it will be saved. This code impacts not only prefab editing mode but also usual scene editing mode because scene objects can be prefab instances as well and potintially can lose changes without a call to PrefabUtility.RecordPrefabInstancePropertyModifications() in the end of changes. I mean all changes should be between undo and property modifications recording calls:

Undo.RecordObject(obj, "Changes");

// Perform changes here

if (PrefabUtility.IsPartOfPrefabInstance(obj))
    PrefabUtility.RecordPrefabInstancePropertyModifications(obj);

PS: I've found this requirement in the official documentation after I'd met a bug with partially losen data.