slight refactoring of widget system code

modules/viz/include/opencv2/viz/widget_accessor.hpp

@@ -1,11 +1,15 @@
 #pragma once
 
-#include <opencv2/viz/types.hpp>
+#include <opencv2/viz/widgets.hpp>
+#include <vtkSmartPointer.h>
+#include <vtkLODActor.h>
 
 namespace temp_viz
 {
-    struct WidgetAccessor
+    //The class is only that depends on VTK in its interface.
+    //It is indended for those user who want to develop own widgets system using VTK library API.
+    struct CV_EXPORTS WidgetAccessor
     {
-        static CV_EXPORTS vtkSmartPointer<vtkLODActor> getActor(const Widget &widget);
+        static  vtkSmartPointer<vtkLODActor> getActor(const Widget &widget);
     };
-}
+}