Add possibility to mark property as required

[jmEvoqua]

Feature Description

It would be nice to mark properties as required, meaning they have to be set from outside. (Like in the Timer)

The use case is, that we have whole navigation trees abstracted away. And there we pipe properties through. The top level component has all properties required for the tree, which will be a lot. So it is easy to miss a property if it gets added in a subpage.

Product Impact

No response

[ogoffart]

That would make sense.
I suggest something like that

export component FooBar {
    required property <int> foobar;
}

Where require is the same as in but must be set when used from Slint.
What happens when FooBar is previewed alone or is top level though? Then it won't get a value.

Does it make sense to give a default value?

// should that be an error?
required property <int> foobar: 42;

Is required the best term?

[jmEvoqua]

I think the option to have a default value is a must for easier development with the live viewer. But it should be limited to the live viewer, meaning a required property with a default value is still required to be set from the outside.
Maybe the placeholder approach #6113 would be a better fit here.

required is also fine for me. It is used in C# for the exact same purpose, or in typescript with a different handling/syntax.
Not saying we should copy these languages but it can be familiar to devs from these languages.

It would also be nice to have required callbacks:

export component FooBar {
    required property <int> foobar;
    required callback clicked();
}