Use a vector of `rtl::Reference<VCLXAccessibleListItem>`
for the children instead of a vector of
`css::uno::Reference<css::accessibility::XAccessible>`
and casting in various places.
Change-Id: I1d0e72a7c844a685d762069002d747cf5e7d02dc
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/156525
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
