Changes to fix NB#80521.

Changes to fix NB#80521.
* src/modest-tny-mime-part.c:
        * Now we recognise a purged attachment as an attachment for
          modest.
* src/maemo/modest-msg-view-window.c:
        * As we now detect purged attachments as attachments, we filter
          here to disallow saving purged attachments.
* src/modest-ui-dimming-rules.c:
        * Remove invalid_account_for_purge as it was wrong, and it's now
          superseeded by _invalid_folder_for_purge.

pmo-trunk-r4217