Allow reference to signals using 'on' handler syntax.

Allow reference to signals using 'on' handler syntax.

This will allow APIs like the following:
trigger: mouseArea.onClicked

However, signal handlers will not be callable from QML:
mouseArea.onClicked() //throws exception

Change-Id: I2ef5cb4e1f3ed4814ef590962391e1b14e3f0c43
Reviewed-on: http://codereview.qt.nokia.com/3683
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>