Add missing constructor and fix access modifiers.

Author: Pavel Kovalenko

src/editors/xrManagedApi/core/fs/FS.cpp

@@ -16,6 +16,7 @@ namespace ManagedApi
 namespace Core
 {
 
+WriterBase::WriterBase(::IWriter* impl) { this->impl = impl; }
 WriterBase::~WriterBase()
 {
     delete impl;

src/editors/xrManagedApi/core/fs/FS.hpp

@@ -62,9 +62,12 @@ public value struct Vector4I
 
 public ref class WriterBase abstract
 {
-private:
+internal:
     ::IWriter* impl;
+private:
     String^ fileName;
+internal:
+    WriterBase(::IWriter* impl);
 public:
     property String^ FileName { String^ get(); }
     ~WriterBase();