Option for importing documents even if open in OO/LO

[peter88213]

This interferes with discarding the document if changed.

• Replace discard_tmp_docs wit an enumeration type.
  0: Discard document only when split
  1: Always discard document
  2: Import even if locked, never discard document
• Select with radiobuttons.
• If sections are split, cancel the import in mode 2.