bnc#863018 WW8 import: fix upper margin of multi-page floating table

A vert orient position was already handled in case a floating table was
imported as a TextFrame, but in case we didn't do that, the vert orient
position was simply lost.

Fix this by importing it as the upper margin of the table (assuming that
the position is relative from the anchor position).

Change-Id: I3e96f3068605fd6313dfb3e55483e1bce6c063a5