localize default notes folder

[korelstar]

closes #486

1. if notes folder was manually set (user setting), then use this folder
2. otherwise, if a folder called "Notes" already exists, use this folder
3. otherwise, use translated folder name and save this as user setting.

The second priority is required in order to be compatible to existing users. Saving as user setting is required, since the translation can change over time, but the notes still have to be accessible without manual interaction.