Add support for importing components, data structures, etc. generated from an earlier Slint compiler invocation

[tronical]

This issue originates from #6196.

Example:

1. Crate A compiles foo.slint, which exports components, globals, data structures, etc. as well as business logic related to foo.slint.
2. Crate B compiles main.slint, which imports foo.slint (for example as a library) and wants to re-use components, etc. as well as the business logic from crate A, without generating foo.slint's types again.

The objective is to enable this workflow for Rust and C++.

In order to implement this, we need to use the Slint library system as well as the ability to associate a Slint library with a Rust module or a C++ namespace, and in addition we need build system support so that cmake or cargo can locate the user's slint library just from the dependencies.

add_library(MyreusableComponent ...)
// Doesn't generate the typical "app" entry but might generate more library/style reusable code
// This also will set various cmake target properties on MyreusableComponent ...)
slint_library_target_sources(MyreusableComponent NAME MyLibrary mylib.slint)

add_executable(my_app ...)
target_link_libraries(my_app MyreusableComponent)
// this needs to look at my_app's linked targets and locate those that contain Slint libraries, to be able
// to locate the header file, library name, etc.
slint_target_sources(my_app main.slint)

// main.slint can do `import { Foo } from "@MyLibrary"`

my-library's build.rs:

fn main() {
    let config = slint_build::CompilerConfiguration::new().as_library("MyLibrary").rust_module("mymodule");
    slint_build::compile_with_config("mylib.slint", config);
}

my-library''s lib.rs`:

pub mod mymodule {
    slint::include_modules!()
}

App's Cargo.toml:

[dependencies]
my-library = { ... }

App's invocation of the slint compiler would use DEP_MY_LIBRARY_SLINT_XXX to locate the path to the .slint library file, name of the library, etc.

It would be tempting to use the links feature of Cargo for this, but that's a hack. The recommended way would be that my-library also has a my-library-build crate that provides API for App's build.rs, to then invoke the slint compiler accordingly.