Changed DBus API to QtDBus

DBusInterface has been almost completely rewritten. Now, instead of checking for DBus messges every
period of time, it uses signal-slots mechanism to listen and react on demand. Code is also a lot
simplier: instead of sending low level DBus messages, reply parameters are simply passed back by
reference.

DBusInterface is now separated from Widget. It's now required to call connectToDBus(DBusInterface&)
to connect both.  This is a first step to provide a separate interface for DBus Message listeners.

Because QtDbus is now used instead of libdbus-1-dev, it's removed from dependency list and is not
compiled anymore.