Add **UI navigation** system

#574

Source/Engine/UI/UIControl.h

@@ -3,15 +3,18 @@
 #pragma once
 
 #include "Engine/Level/Actor.h"
+#include "Engine/Scripting/ScriptingObjectReference.h"
 
 /// <summary>
 /// Contains a single GUI control (on C# side).
 /// </summary>
 API_CLASS(Sealed) class FLAXENGINE_API UIControl : public Actor
 {
-DECLARE_SCENE_OBJECT(UIControl);
-public:
+    DECLARE_SCENE_OBJECT(UIControl);
+private:
+    ScriptingObjectReference<UIControl> _navTargetUp, _navTargetDown, _navTargetLeft, _navTargetRight;
 
+public:
     // [Actor]
 #if USE_EDITOR
     BoundingBox GetEditorBox() const override;
@@ -20,7 +23,6 @@ public:
     void Deserialize(DeserializeStream& stream, ISerializeModifier* modifier) override;
 
 protected:
-
     // [Actor]
     void OnParentChanged() override;
     void OnTransformChanged() override;
@@ -28,4 +30,11 @@ protected:
     void OnEndPlay() override;
     void OnOrderInParentChanged() override;
     void OnActiveInTreeChanged() override;
+
+private:
+#if !COMPILE_WITHOUT_CSHARP
+    // Internal bindings
+    API_FUNCTION(NoProxy) void GetNavTargets(API_PARAM(Out) UIControl*& up, API_PARAM(Out) UIControl*& down, API_PARAM(Out) UIControl*& left, API_PARAM(Out) UIControl*& right) const;
+    API_FUNCTION(NoProxy) void SetNavTargets(UIControl* up, UIControl* down, UIControl* left, UIControl* right);
+#endif
 };