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

AssetStudioFBXNative/asfbx_context.cpp

@@ -0,0 +1,33 @@
+#include <fbxsdk.h>
+
+#include "asfbx_context.h"
+
+AsFbxContext::AsFbxContext()
+{
+	pSdkManager = nullptr;
+	pScene = nullptr;
+	pTextures = nullptr;
+	pMaterials = nullptr;
+	pExporter = nullptr;
+	pBindPose = nullptr;
+}
+
+AsFbxContext::~AsFbxContext()
+{
+	framePaths.clear();
+
+	delete pMaterials;
+	delete pTextures;
+
+	if (pExporter != nullptr) {
+		pExporter->Destroy();
+	}
+
+	if (pScene != nullptr) {
+		pScene->Destroy();
+	}
+
+	if (pSdkManager != nullptr) {
+		pSdkManager->Destroy();
+	}
+}