Regression from commit 01fc08c0
(fdo#85554 SwXShape: fix getting ZOrder property when doc contains
TextBoxes, 2014-11-20), the problem was that we returned wrong ZOrder of
shapes inside group shapes.

In SwXShape::getPropertyValue(), pObj points to the Writer-interfacing
outermost group shape in case of shapes contained by group shapes, while
GetSvxShape() gives access to the real shape. Given that TextBoxes are
only possible at the highest level (and not inside group shapes), just
check if the two pointers are the same: when not, then no need to
convert anything.

With this, child shapes get back their original ZOrder -- before in case
the group shape had ZOrder=0, all its child shapes had ZOrder=0 as well.

Change-Id: I9c4097154130cd04f6ab2f2082abafc1d4333872