Aha, now we know that the reason for the defensive programming
in lcl_AssureFieldMarksSet() was that there are actually 2
different use-cases for it: usually a new mark is inserted,
so there are no dummy characters and they must be inserted.

However when copying text, the dummy characters are copied too,
so they must not be inserted, or we get duplicate fieldmarks.

This also reverts commit d4036d3a
which fixed the problem only for CHECKBOX_FIELDMARK in a
different way.

(regression from bb069fe7)

Change-Id: I3c99b8c6d720951655198e682018794337859373