Add first version of Material Graph Common Interface (naming is pretty temporary)

.gitignore

 Assets/UnityHDRI/Gareoult/GareoultWhiteBalanced.exr
 Assets/GraphFramework/*
 Assets/NewUI/*
-Assets/UnityShaderEditor/*
+Assets/UnityShaderEditor/*
+Assets/MaterialGraphInterface/*

_ExternalSymLink.bat

 mklink /J "%cd%\Assets\UnityShaderEditor" "%cd%\External\MaterialGraphProject\Assets\UnityShaderEditor"
 mklink /J "%cd%\Assets\NewUI" "%cd%\External\MaterialGraphProject\Assets\NewUI"
 mklink /J "%cd%\Assets\GraphFramework" "%cd%\External\MaterialGraphProject\Assets\GraphFramework"
+mklink /J "%cd%\Assets\MaterialGraphInterface" "%cd%\External\MaterialGraphProject\Assets\MaterialGraphInterface"
 pause