svx: fix VML export of rectangles imported from drawingML

This wasn't a problem before, as EnhancedCustomShapeTypeNames wasn't
involved for the export of rectangles, as those were handled in sw
directly.

CppunitTest_sw_ooxmlsdrexport's testFdo69636 is a reproducer for this
problem, the VML fallback part was empty due to this.

Change-Id: Ib2622fe05ddaa66bb3231977e30a8a1ca8748001
VML export: handle textbox text

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