interface should allow adding an unexposed setter

[zeenix]

Typically you want properties to be read-only on the bus but settable from the service process itself. When you do set them manually, you need to write a bit of a complicated code:

struct MyIface(u32);

#[interface(name = "org.myiface.MyIface")]
impl MyIface {
     #[zbus(property)]
     async fn count(&self) -> u32 {
         self.0
     }
}

let iface_ref = connection
    .object_server()
    .interface::<_, MyIface>(path).await?;
let mut iface = iface_ref.get_mut().await;
iface.0 = 42;
iface.count_changed(iface_ref.signal_context()).await?;

It would be great if allowed adding a non-exposed setter in interface that sets the value and each time it's called, it handles the signal emission for you. So the property setting code above becomes:

let iface_ref = connection
    .object_server()
    .interface::<_, MyIface>(path).await?
    .set_count(42)?;