per-file-settings: encryption password

[Encryption](https://github.com/gsantner/markor/pull/887) is an awesome start, thank you @opensource21 and @gsantner for getting this in. For me, this was a critical feature.

The existing feature addresses the data at rest encryption. **This feature request is to make "locking" easy.** This allows one to let someone else use their phone for a few minutes, unsupervised, without fear that private notes can be read. This can be accomplished in one or more of these ways:
- by having Markor forget the password on a configurable timeout (eg: after 5mins)
- by having Markor forget the password when exiting the app
- by placing a "forget" button on the main app screen

Is one or more of the above an easy feature to implement? Happy to hash this out further, provide examples of other open source apps that do this, etc. Thanks much for considering!

May I also take a moment to say how awesome Markor is. It's exactly what I was looking for. Thank you for building, sharing, and maintaining it @gsantner!

Start with the encryption implementation linked in pull request #887 and trace how the password is retained across the app lifecycle. Clarify with maintainers which forgetting trigger and configuration are in scope; done means the password is forgotten after the chosen trigger while existing encryption behavior remains intact.

Written by the indexing model from the issue text.