EscherPropertyContainer::CreateTextProperties: do not write
ESCHER_Prop_txflTextFlow for custom shapes.  This happens to export the
rotation properly for the WW8 bugdoc of n#404221, as well as the PPT at
fdo#58204 and both PPTs at fdo#34897, one of which (mangle.ppt) was not
handled properly on master currently.  The author disclaims having any
actual knowledge about rotation properties in MSO files.

(regression from 24dd42f3)

Change-Id: Id3dcfb6221d031cdabb7f543b68752b467d9e1a6