Object const-correctness.

Const function shouldn̈́'t return anything with which it would be possible
to change object's internal state:

 * It is possible to get non-constant pointer to itself from
   parent().children() array, so parent() cannot be const function.
 * The same applies to scene().
 * It is also possible to get non-constant pointer to itself from
   children()[0].parent(), so children() also cannot be const function.

src/Object.cpp

@@ -79,7 +79,7 @@ Object::~Object() {
         delete *_children.begin();
 }
 
-Scene* Object::scene() const {
+Scene* Object::scene() {
     /* Goes up the family tree until it finds object which is parent of itself
        (that's the scene) */
     Object* p = parent();

src/Object.h

@@ -65,13 +65,13 @@ class MAGNUM_EXPORT Object {
          * @brief %Scene
          * @return If the object is not assigned to any scene, returns nullptr.
          */
-        Scene* scene() const;
+        Scene* scene();
 
         /** @brief Parent object */
-        inline Object* parent() const { return _parent; }
+        inline Object* parent() { return _parent; }
 
         /** @brief Child objects */
-        inline const std::set<Object*>& children() const { return _children; }
+        inline const std::set<Object*>& children() { return _children; }
 
         /** @brief Set parent object */
         void setParent(Object* parent);