Previously, we always exported the text of the shape itself. Bring the
VML export in sync with the drawingML export, where we only do that if
the shape doesn't have an associated textbox -- if that's the case, then
export the textbox's text instead.

CppunitTest_sw_ooxmlsdrexport's testFdo69636 is a reproducer for this
problem, the VML assert failed because of the lack of this.

Change-Id: Icb236579da4e3b74e983a95aa5675fed7862d1e1