Fix signal handlers for signals with default arguments.

Fix signal handlers for signals with default arguments.

For cloned signals, connect to the index of the original. This was a
regression caused by the switch to QQmlNotifierEndpoint for signal
handlers. This change also makes parameters with default arguments
available from QML, while previously they were unavailable.

Change-Id: I1feb3412c3e9b0f2a5d6644c404c56d53c5544ac
Reviewed-by: Martin Jones <martin.jones@nokia.com>