dbusmonitor (breaking API): added dbusTree parameter to __init__
The dbus monitor class used to monitor always for all values. This
change is one of the few changes necessary to make dbusmonitor also
usefull for other scripts that just want to monitor battery soc for
example.
dbusmonitor: added functions get_value and get_service_list
vedbus: added VeDbusService.add_mandatory_paths(...)
dbusmonitor: little refactoring, no functional changes