gdbus-wrapper: Fix emit after unexport

First implement support for exporting over multiple connections, which was missing in the original implementation.

Then add overrides for g_dbus_interface_skeleton_unexport () and g_dbus_interface_skeleton_unexport_from_connection () that ensure if the interface is being unexported from the last connection any pending property emissions are not emitted after the connection is dropped.

closes #332 (closed)