Replace C++/CLI components with P/Invoke components (#562)

* Replace C++/CLI components with P/Invoke

* Deleted C++/CLI projects

* Use Utf8StringHandle to marshal UTF-8 strings

* Use plaform-default calling convention

* Handle DLL preloading on Linux and macOS

* Change intermediate and output directories of native projects

* Improve P/Invoke documentation

AssetStudioFBXWrapper/FbxDll.cs

@@ -0,0 +1,10 @@
+namespace AssetStudio.FbxInterop
+{
+    internal static class FbxDll
+    {
+
+        internal const string DllName = "AssetStudioFBXNative";
+        internal const string FbxsdkDllName = "libfbxsdk";
+
+    }
+}