QFlags support

[LeonMatthesKDAB]

          We need to figure out a plan for QFlags maybe something with https://crates.io/crates/bitflags and https://crates.io/crates/flagset etc

Originally posted by @ahayzen-kdab in #555 (comment)

[nicolasfella]

I played around with this and it looks rather easy.

QFlags is a class with one int member.

On the Rust side we only need

bitflags! {
    #[repr(C)]
    pub struct MyFlags: u32 {
        const A = 0x01;
        const B = 0x02;
    }
}

plus the usual ExternType boilerplate, and then it just works

[LeonMatthesKDAB]

@nicolasfella thank you for investigating.
That seems like a decent solution.

Note that this would be slightly different from how it works in Qt, where QFlags is a generic, which it wouldn't be in Rust.
But that's probably okay, as generics in CXX come with their own set of issues.