FRED has an autosave feature but it wasn't invoked in too many places. This adds autosaving whenever a major dialog is closed, unless it was closed by hitting Cancel. NB: Autosave is not yet implemented in qtFRED.
Also, `addvariabledlg` incorrectly referred to `OnOk` instead of `OnOK` and `CMessageEditorDlg` incorrectly referred to `OnOK` instead of `OnOk`, so this fixes those and distinguishes `OnOk` by renaming it `OnButtonOk` to be consistent with the event editor and other dialogs.