Made sure items with preferred width of 0 could also stretch

If no stretch factors were specified, we used the preferred size as a
stretch factor. Obviously, that didn't work if the preferred size was
actually 0.

This patch works around this by actually setting the stretch factor to
1.0 if this is the case.
This should work fine in most cases, except for the case where there
are also other items with a preferred size close to 0.
In this case, the item with preferred size 0 will just grow
faster than an item with e.g. preferred size 0.1.

Task-number: QTBUG-31217

Change-Id: I966455da0bdd00308591c7f7cfbc4e134d423e57
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>