MonitorManager was inheriting from MetaDBusDisplayConfigSkeleton, this was
causing introspection to see this like a GDBus skeleton object exposing to
clients methods that were not required.

Also, this required us to export meta_dbus_* symbols to the library, while
these should be actually private.

So, make MetaMonitorManager to be just a simple GObject holding a skeleton
instance, and connect to its signals reusing most of the code with just few
minor changes.

!395