Disallow signal-handler-specification for non-signal methods

Disallow signal-handler-specification for non-signal methods

Previously, the code which checked whether a signal handler
specification was valid was incorrect, in that it only checked that a
method of the appropriate name existed (rather than specifically a
signal method).

This commit ensures that the appropriate code is updated to check the
method for signal-ness.

Task-number: QTBUG-26223
Change-Id: I306f6622aaa710d86c01d5bbc5146eecce7cf2c3
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>