* Added "Export as Image" menu.

**Bugfixes:**
* Cylinder tesselation broke existing models which are using cylinders
  for e.g. captured nut slots and are dependent on the orientation not
  changing.
* DXF output couldn't be imported into e.g. AutoCAD and Solidworks after updating
  to using the AutoCAD 2000 (AC1015) format. Reverted to the old entity-only output,
  causing LWPOLYLINES to not exported allowed anymore.