ObjectModel: add API for dynamic changes

ObjectModel: add API for dynamic changes

Following the ListModel API:
- object get(index)
- append(object)
- insert(int index, object)
- move(int from, int to, int n)
- remove(int index, int n)

[ChangeLog][QtQml][ObjectModel] Added get(), append(), insert(), move()
and remove() methods.

Change-Id: I592e55b7c4c933a1100191bf5a9405944b347172
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>