Optional knob to shuffle wl_signal_add ordering

A very common mistake in my experience is to implicitly rely on wl_signal_emit invocation order. When adding two listeners A and B, libwayland will consistently call first B then A. It's easy to miss this and write logic which doesn't account for the case where A is invoked then B.

It would be handy to have some kind of debug flag to randomize wl_signal_add insertion order, or wl_signal_emit invocation order.