Move #[signal] and #[inherit] to extern "RustQt" block

[LeonMatthesKDAB]

See target API in #555

This would unify use of inherit, and make it the same syntax, no matter whether you're inheriting a normal function or a signal.
Also, signals would no longer be an enum, but rather just functions, which also makes our code generation a lot simpler.

[ahayzen-kdab]

First stage of this would be to move

    #[cxx_qt::qsignals(RustSignals)]
    pub enum Connection<'a> {
        /// A Q_SIGNAL emitted when a connection occurs
        Connected {
            /// The url for the connection
            url: &'a QUrl,
        },
    }

to

    #[cxx_qt::qsignals]
    extern "C++" {
        fn connected(self: Pin<&mut qobject::RustSignals>, url: &'a QUrl);
    }

Which would bring the API to the same as the inherit block. Then the next stage would be to move both the inherit and signals into the extern "RustQt" block with attributes on each of the signals instead.

And by doing this initial move it also unblocks #556 as then we won't have the enum so properties can generate signals in the same function way.