fontinfo.plist import/export v4. update for PR 215

see https://github.com/metapolator/metapolator/pull/215
Oh, I see. you want to use `this._fontinfoIsInitialized` so that you know if there is a `fontinfo.plist` on disk or not?
How about raisinng an error in the getter? A `KeyError` would be an option.