Trade: proper const overloads.

This is one of these dark corners.

src/Magnum/Trade/ObjectData2D.h

@@ -94,6 +94,7 @@ class MAGNUM_EXPORT ObjectData2D {
 
         /** @brief Child objects */
         std::vector<UnsignedInt>& children() { return _children; }
+        const std::vector<UnsignedInt>& children() const { return _children; } /**< @overload */
 
         /** @brief Transformation (relative to parent) */
         Matrix3 transformation() const { return _transformation; }

src/Magnum/Trade/ObjectData3D.h

@@ -96,6 +96,7 @@ class MAGNUM_EXPORT ObjectData3D {
 
         /** @brief Child objects */
         std::vector<UnsignedInt>& children() { return _children; }
+        const std::vector<UnsignedInt>& children() const { return _children; } /**< @overload */
 
         /** @brief Transformation (relative to parent) */
         Matrix4 transformation() const { return _transformation; }