handle Unix signals in the QML runtime

There are various reasons you might want to reload a QML application, and it would be convenient to be able to do that without killing the process and starting another. (For live-coding for example, or when you know that some remote resource has changed.) It comes naturally to try to send SIGHUP to the qml executable to see what happens, and so far that simply makes it quit. So what if we install a signal handler to make it reload instead?

http://doc.qt.io/qt-5/unix-signals.html describes one way that signal handlers can be implemented in Qt applications. I wonder if it really needs to involve a socket pair.

We could think about doing other things with other signals too, if there are use cases.