Invalid selections in form editors

[michael]

Search for a more flexible solution to #358

A possible solution could be introducing a new selection type InvalidSelection, which has no correspondence in the document model, but can be used in the browser for instance to copy and paste content.

[obuchtala]

For now, I just create a null selection in these cases. I.e., it is not possible to copy from a form editor outside of fields.
This proposal is rather a big thing, as we need to introduce the isInvalid() check at many places.
So let's wait.