Collection made generic & type to Group.children

[alper-batioglu]

Group.children if I am not mistaken is the collection of items added by the Group.Add method. So they are TwoElements.
With this assumption I made Collection generic. (with default of any in order to prevent affecting other usages)
And also defined Children type as a collection of TwoElements. And also Group.children as Children.

this made usages possible like,
this.root.children.forEach(child => this.root.position.blablabla

I am still thinking of
export class Children extends Collection<Shape> ...
but children is filled by Add and it accepts TwoElement; so I think i will stick with that.

[jonobr1]

Thank you for the improvements!!