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_morph_context.cpp

@@ -0,0 +1,9 @@
+#include "asfbx_morph_context.h"
+
+AsFbxMorphContext::AsFbxMorphContext()
+{
+	pMesh = nullptr;
+	lBlendShape = nullptr;
+	lBlendShapeChannel = nullptr;
+	lShape = nullptr;
+}