Fallback behavior for documents whose generator provides native annotation editing support without saveAs support

The only affected generator is poppler. Note that:
Document has /Encrypt <=iff=> SaveInterface supportsOption(SaveChanges) is false

This patch enforces the following behavior (and warns the user the first time
he edits an annotation).

- If the document has /Encrypt, warn that "Save as" is not available, but it's
possible to export as okular archive. New annotations will be automatically
saved to XML as usual.
Note that the previous patch already made all existing annotations uneditable,
because there's no way to save them.

- If the document has no /Encrypt and there are existing external annotations,
warn that changes won't be saved automatically. The user needs to "Save as" or
changes will be lost.

- If the document has no /Encrypt and there aren't existing external
annotations, don't show any warning. New annotations will be automatically saved
to XML as usual and to file if "Save as" is pressed.