#733

Also disallow deletion of only non-permanent page.

#700

This should allow users to notice, when errors happen and possibly 
report them.

- Image
- Hotspot-Image
- Audio
- Video
- Geometry
- DragNDropValueObject (imgFileName, audioFileName)
- LikertRowElement (imgFileName, audioFileName)

- remove 'own' as possible connection

- remove 'none'

#691

- Due to the width of the scrollbar, text areas change their inner width
under Chromium when the scrollbar is displayed. The use of offsetWidth
instead of contentRect ignores the presence of the scrollbar. The
calculation is therefore less accurate, but sufficient for estimating
the number of rows

#662

#643