[mediator/unity/client] add new test

[Egliss]

The PR dependency to #61

Summary

the PR add the Unity Project used by #61 's test and fix tiny bug.

• Update UnityProject
  • Update Unity ver v2018.3.0b12-> v2018.4.3f1 (LTS)
  • Add DefaultAssetExporter processable asset
    • new scene +1
    • new model +2
  • Add some directory
• Update sgmed.config will call new scene
• Fix exporter's lacked asset check

#61 does not commiting in the PR

Review files

• packages/scene-graph-unity/client/src/exporter/scene/DefaultSceneExporter.ts
• packages/scene-graph-unity/client/example/sgmed.config.js

Check

• tslint
• Test SampleScene and MeshRendererScene
  • Check DefaultAssetExporter's error([mediator/unity/client] Fix relative path #61 ) with windows
    • Check apply [mediator/unity/client] Fix relative path #61 then DefaultSceneExporter's error removed
  • Check DefaultSceneExporter's error removed

TODO

• Update README.md

[Egliss]

sometimes assets count is zero.