DBus signaller will combine duplicate signals for same object path

This change should improve performance when multiple properties change on an object per
round.
Fixed issue with Murphy plugin using wronge type for certain Properties.

Murphy tries to create a basic type even if the type is already registered.
This wouldn't typically be a problem, except that certain types are enums
which murphy doesn't know about.

The solution is to use VehicleProperty::getPropertyTypeForPropertyName if the
property has already been registered by another plugin or by AMB core itself.