[BUG] property macro #[must_use] since 0.18.2

[AaronErhardt]

Bug description

After a cargo update which bumped glib-macros to 0.18.2, I've got several of clippy warnings in Relm4:

warning: this method could have a `#[must_use]` attribute
   --> relm4/src/binding/bindings.rs:59:17
    |
59  |                 #[property(get, set)]
    |                 ^ help: add the attribute: `#[must_use] #`
...
101 | binding!(StringBinding, "StringBinding", String, imp_strin...
    | ------------------------------------------------------------ in this macro invocation
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#must_use_candidate
    = note: this warning originates in the macro `binding` (in Nightly builds, run with -Z macro-backtrace for more info)

It seems like the macro is using quote_spanned since #1149, which makes clippy think that the user forget to add a #[must_use] if you enabled the clippy::must_use_candidate lint for your project. quote_spanned might help to improve the error reporting, but it can also trigger unexpected clippy lints.

To solve this, either #[must_use] should be added to the generated methods or the spans have to be adjusted carefully to make it clear for clippy that those tokens were produced by a macro.

Steps to reproduce

1. Use the property macro
2. Enable the clippy::must_use_candidate lint
3. Rust cargo clippy