I've an internal RH document which crashes in a11y when a paragraph with a
graphic and a drawing frame in it is deleted.

The SdrObject is removed and deleted, but when
SwAccessibleContext::DisposeChild is called the object does not pass IsShowing
so its not removed from the accessibility map. Leaving an entry in the map
pointing to a deleted SdrObject

So here take the route-one approach of always removing from the map
accessibility children which depend on a SdrObject which is getting deleted,
whether or not it is inside the visible area at the moment.

The real change here is to SwAccessibleContext::DisposeChild and to
SwFrame::RemoveDrawObj

Change-Id: I764cd54d6216d233756f52b5be66c80737b5e38d
Reviewed-on: https://gerrit.libreoffice.org/26824Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>