Turns out that for normal TextFrames and rectangular drawinglayer
shapes, the engine is written in ODF implicitly. Use the same trick to
describe if the shape content should be rendered by SW or editeng.

This reverts 9d310ecf (sw textboxes:
implement ODF import/export, 2014-06-02).

Change-Id: Id8afa6368bedcd293e578244abb58e18949bec8d