Add signal-composer event support to libqtappfw

It would simplify changes to the dashboard, tbtnavi, cluster gauges, and potentially other Qt applications if libqtappfw had support for subscribing to and receiving events from signal-composer.  Initially, support should be focused on the upcoming event.vehicle.speed and event.engine.speed events that will be added to signal-composer to abstract the underlying CAN messages.  Following that, at least some of the in the works events for the steering wheel buttons will need to be supported.