Make Repeaters and Positioners work together.

This commit fixes to bugs, one in Repeaters on in Positioners:
- Repeaters added items as their own children instead of the children of the
parent like it is done in Qt Quick. That's needed to be able to position them
using Row, etc.
- Positioners also positioned items that has a width or a height of 0. Qt Quick
Positioners don't do that. This commit fixes that.