This avoids problem with the selected index when moving pages.

#737

Not closing the menu does not make sense when moving or deleting pages.
It was only useful to avoid closing when clicking on the menu without 
clicking a control. Losing this is acceptable.

Also disallow deletion of only non-permanent page.

#736

IDs can and will be generated by the element constructors.

#716

This may happen when changing the Clote document property.

Also move cloze-code into Cloze and no longer have it in a general 
purpose service.

This is done within the element's logic.

#700

It suddenly works without that. No idea why.

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

It now uses the page index the menu (that was used to delete) belongs 
to, instead of the selected page index.

- concerns media elements

- functionality will be implemented later

There always has to be at least one row and column.

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

#695

This way the table fits better into the dialog dimensions.

This is meant for source of the audio.

Just ignore the color in the component.

- editor and player have their own services to handle the errors