Work to complete implementation of purge attachments operation

Work to complete implementation of purge attachments operation
(fixes NB#61240).
* src/widget/modest-msg-view.[ch]:
        * Added API function to get all the attachments in
          the view, and not only the selected ones.
* src/widgets/modest-attachment-view.c:
        * Support for showing purged files improved (still pending
          a proper logical id).
* src/widgets/modest-msg-view-window.[ch]:
* src/modest-ui-actions.[ch]:
        * Added support for purging attachments of a message from the
          main window, with lots of exception conditions (trying to
          remove messages with all attachments already purged, etc).
        * Added confirmation dialog for removing attachments from
          main window.
* src/maemo/modest-main-window-ui.h:
        * Attach remove attachments action in main window.
* src/widgets/modest-msg-view-window.[ch]:
        * Now you cannot save or view a purged attachment.
        * Now you can choose in ..._remove_attachments to remove
          only selected attachments or all the available attachments.
        * Skip already removed attachments
        * More informationbanners for purge operation.
        * Now the message is correctly reloaded after being purged.
* src/widgets/modest-ui-dimming-rules.[ch]:
        * Added a lot of dimming rules for removing attachments both
          from main window and msg view window.

pmo-trunk-r2478