o use layer of child shapes to find out the layer of a group shape.
  This is done as the group does not allow to save its layer in odf

This fixes loading and saving of grouped shapes. Before the fix the
group was always added to the default layer. Now the group is added to
the layer it is was in when saving.

OK to backport to 2.1? I think it is a important fix as some information
is lost during save/load of the document.

CCMAIL: koffice-devel@kde.org

svn path=/trunk/koffice/; revision=1047373