allow non-unit signal return values

As per #39 some signals do not return () but rather Inhibit(bool) or similar flags.

This PR introduces a IntoSignalReturn trait that has a blanket implementation for converting types into (). This trait is then used to create the return value of the signal callbacks. It will automatically convert any message to (), and allows the user to implement custom conversions, as follows:

impl IntoSignalReturn<Inhibit> for Message {
    fn into_signal_return(&self) -> Inhibit {
        Inhibit(false)
    }
}

My main concern is if this handles all cases, or needs special handling for subcomponents as @LiHRaM seemed to suggest. @bodil any input?

bodil / vgtk

allow non-unit signal return values #70