Update Qt demo applications to use VIS

Several of the Qt demo applications that were previously getting signals via the signal-composer abstraction provided by libqtappfw need updates to use the new Vehicle Information Service (VIS) signalling provided by KUKSA.val and corresponding new support for it in libqtappfw.  The known affected demo applications and their signalling requirements are:

• dashboard: vehicle and engine speed
• cluster dashboard: vehicle and engine speed, assorted cruise control signals
• ondemandnavi: route destination and current position publishing, possibly routing start/stop publishing
• tbtnavi: route destination, current position?, possibly routing start/stop, display mode switching
• mediaplayer: next and previous track, mode/source selection

Volume control signals from the demo steering wheel setup had been handled in the agl-service-audiomixer binding and not in an application, connecting those will like need to wait until its replacement appears.