Add UNIX signal handler integration to the GLib event dispatcher

QCoreApplication has an internal function called QCoreApplication::watchUnixSignal() which makes it easier (and safer) to handle UNIX signals inside Qt. When using this function with the GLib event dispatcher, the handler is registered and called correctly (updates the sig_atomic_t variable), but the integration with the event dispatcher is missing such that the Qt signal, QCoreApplication::unixSignal, is never emitted. This would be nice to have working.

It would also be useful if this was a publicly documented function instead of internal.