[Open] object api for returning child index of the desired parent

[Calamath]

There are several object APIs related to Control.

* GetChild(*luaindex* _childIndex_)
** _Returns:_ *object* _childControl_

* IsChildOf(*object* _desiredParent_)
** _Returns:_ *bool* _isChild_

However, there is no way to get the childIndex of a specific child control associated with the parent control itself.
Therefore, I would be grateful if you could add the following as an object api for Control.

* GetChildIndexOf(*object* _desiredParent_)
** _Returns:_ *luaindex* _childIndex_


I want to insert a child control from the control pool to the flex container at the next index after the specific child control.

We have SetParent and InsertChildBefore, and I would appreciate having InsertChildNext as well.

* InsertChildNext *protected-attributes* (*object* _childControl_, *object* _previousChild_)

I know using InsertChildBefore twice would do the same thing, but I think it would be wasteful to have to do the layout calculation twice.

Can you please kindly consider my proposal?
Thank you.

- Calamath