Add a new ToDo MVC example

[FloVanGH]

I'm working right now on a new version of the Todo example with a much nicer design (still usable with all styles) and based on the Model View Controller pattern.

First goal of this in first place is to have a small example with two views to show how work with the MVC pattern on Slint for all supported languages (Rust / C++ / Node / Python). I first will start with the Rust version. And want to add additional languages with following PRs. With the Python support I would need @tronical 's help ;-). But after first PR is finished.

Second goal is to has a good looking example for a lightweight real live application, that works also good on mobile.

Tasks

• [ ] improve accessibility of the ui (see https://github.com/slint-ui/slint/pull/5396#issuecomment-2163952542)
• [x] Implement Rust example with mock data
• [ ] Serialize / De-serialize data and android port
• [ ] C++ version
• [ ] TS version
• [ ] Python version
• [ ] Remove the old todo example after all previous tasks are done

[tronical]

I suggest to replace pub due_date: i64 with std::time::SystemTime as data type instead. That way you have a data type that chrono supports (implements From<SystemTime> for chrono::DateTime) and you can make it relative to the epoch if necessary. But for example if you end up backing your model with a database, you wouldn't store an i64 in the database, you'd use the database' date time data type anyway.

[tronical]

Another suggestion: replace unwrap() in main with Result.