bnc#880763: PPTX import: wrong z-order becuause of wrong import order

importExtDrawings() must be called as soon as possible,
before parser starts to parse the next shape.
Call it when graphicFrame tag is closed. This tag include
the reference to the SmartArt.

Plus fix up import tests.

Change-Id: I9e8d54c2b1afeb78a1122390dc4982d580c152ae