live-reload of .slint file for rust compiled app

[ogoffart]

This will generate API that calls the interpreter behind the scene.
And it will set a file system watcher so reload automatically

Can be tested by running an app like so:

SLINT_LIVE_RELOAD=1 cargo run -p gallery --features=slint/live-reload

SLINT_LIVE_RELOAD=1 is needed at build time so the rust compiler generate the right thing, the slint/live-reload feature will be used to enable the runtime for the live-reloading

• Since only the public API needs to be compiled by the rust compiler, the rust compilation is faster.
• The automatic reloading feature works Ok, but can lead to unexpected states. But the application can still be restarted without compiling.
• If the interface changes in an incompatible way (deleted properties or callback or to incompatible types) then the app will panic.

Rust part of #4128

[tronical]

Looks good at first glance (just glanced because it's still a draft). Quite elegant and minimal.

One missing item is perhaps to run the test driver with this also in the CI, so that we catch any accidentally regressions in the new code generator.

[ogoffart]

The code here is something like that:

    property<[{name: string, account: string, score: float}]> model: [
//   ...
    ];


    for person[i] in model: TouchArea {
        // ...
        clicked => {
            person.score += 1;
        //...

The test does set a ModelRc<(slint::SharedString, slint::SharedString, f32)> to the property, then click, then expect the model to be changed.

But the ModelRc needs to be mapped. Since Value doesn't implement From<> for tuples (it can't, really, how would they know the names of the fields), Then the code generated there do the conversion:
https://github.com/slint-ui/slint/pull/8837/files#diff-70fcd8b8c35cc6705d940dbda1e7664aa8d23a78c32de31466c66d2ece515011R370-R372

                let sp::live_reload::Value::Model(model) = v else { return sp::Err(()) };
                sp::Ok(sp::ModelRc::new(model.map(|x| #conf_fn(x).unwrap_or_default())))

But model.map returns a MappedModel that doesn't implement set_row_data. Cf #5290

[tronical]

I suggest to also add the run-if-changed for the env variable to trigger rebuilds correctly.

[tronical]

Suggested change

	## This feature allow to reload the .slint files at runtime, and reload it whenever the files are modified on disk.
	## In order to use this, you will also need to define the `SLINT_LIVE_RELOAD` environment variable while
	## doing the slint compilation.
	## Enable this feature to reload the .slint files at runtime and reload it whenever the files are modified on disk.
	## For this feature to work, it's also required to set the `SLINT_LIVE_RELOAD` environment variable during
	## application compilation.

[tronical]

Suggested change

	## Since this is a debug-only feature, it is not recommanded to add this feature to your Cargo.toml.
	## Instead, you can use the `--features` command line argument like so when building your app:
	## ```bash
	## SLINT_LIVE_RELOAD=1 cargo build --features slint/live-reload
	## ```
	## This is a feature for debugging and development. It's not recommended to add this feature to your Cargo.toml.
	## Instead, use the `--features` command line argument like so when building your app:
	## ```bash
	## SLINT_LIVE_RELOAD=1 cargo build --features slint/live-reload
	## ```