Add dimming for save to drafts menu action (fixes NB#66476).

Add dimming for save to drafts menu action (fixes NB#66476).
* src/widgets/modest-msg-edit-window-ui.h:
        * Add email menu ui action for checking dimming rules.
* src/maemo/modest-msg-edit-window-ui-dimming.h:
        * Add dimming rule for save to drafts to editor.
* src/modest-ui-dimming-rules.[ch]:
        * Added dimming rule implementation for save to drafts in
          editor. If message is not marked as modified, we don't dim.

pmo-trunk-r3353