Shorthand to directly wrap functions on the inner Rust struct

[LeonMatthesKDAB]

Currently, if you want to expose a function on the inner Rust struct to C++/QML, you always have to write a wrapper function outside the bridge.

E.g.:

#[cxx_qt::bridge]
mod qobject {
extern "RustQt" {

#[qobject]
type MyObject = super::MyObjectRust;

fn my_fun(self: &MyObject) -> i32;

}
}

impl qobject::MyObject {
    fn my_fun(&self) -> i32 {
        self.rust().my_fun();
    }
}

We could consider automating this if you provide a given attribute to the declaration (e.g. #[inner]).

Example:

#[cxx_qt::bridge]
mod qobject {
extern "RustQt" {

#[qobject]
type MyObject = super::MyObjectRust;

#[inner] // auto-generates the wrapper outside the bridge
fn my_fun(self: &MyObject) -> i32;

}
}

Note that this may cause issues with Result<>, as the Error type wouldn't necessarily be known.