Only one signal per object?

[Andrej Mitrovic]

I think I know what's going on,

getCallable() doesn't check for the storage class of a function, and
does a cast based on the parameters and the return type when it's
adding a new handler. The handler is later casted to a function type
with no ref parameters, and this is where things probably cause
problems.

I'll have to devise something more sophisticated.