Make out properties public to the native language

[LeonMatthesKDAB]

Feature Description

@ogoffart @tronical this issue was one of the "low hanging fruits" we identified in our recent chat.
Again, let me know if I misunderstood anything.

When implement the backend for a given component or global, it would be quite useful to have access to the out properties of the slint component from the native backend.
To quote the docs:

out: An output property that can only be set by the component. It’s read-only for the users of the components.

In my opinion, a native backend that is tightly coupled to a global (or once #2031 is implemented a component), should count as "part of the component" as far as .slint code is concerned.
The backend should be able to entirely drive the properties from native code.
By marking them as out, instead of in-out, the properties are protected from accidental overwrites in other slint code.
This ensures that the native backend always has control over the property content.

However, at the moment this is not possible, as an out property does not have a setter in native code.
Simply generating the setter for out properties would solve the issue.

Product Impact

No response

[tronical]

So this only affects exported components/globals, for which we'd make out properties accessible via the API.

[LeonMatthesKDAB]

Yes, this should not include any changes for the slint language itself, only for the API generated in the native language.
(Which of course is only generated for exported components/globals)