qmlSignalParameters warning is unhelpful

If the type is unknown,  the warning is attached to the handler; but a user can't do anything at this point. It should at least point to the place where the signal is defined in addition – if at all. If the parameter is not used at all in the handler, it can be questioned whether there really ought to be a warning in the first place.

Moreover, for QML defined signals, we should warn about invalid type annotations instead (and use didYouMean to handle potential typos).