SignalHandlerProperty allows editing convenient editing of signal
handlers like onMousePress in the model.
The interface is analogous to BindingProperty.

Since mos views do not care about SignalHandlerProperties and
they should not be mixed up with BindingProperties they are different
types.

I also added the signalHandlerPropertiesChanged() notifier to AbstractView.

Change-Id: I68bc7d2c5d3b991944e8f8d698212a1dfef218bf
Reviewed-by: Marco Bubke <marco.bubke@digia.com>