Add source QObject to QQuickAction's triggered signal

Based on a suggestion from Gabriel de Dietrich. Knowing the source
object that triggered an action is useful for analytics purposes.

Change-Id: I11f62214fa669a91769a0ce25b0c11c0fc0635d0
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>