core::dbus::traits::Service<T>::interface_name() seems to be used for both interface names and bus names

[James Henstridge]

the core::dbus::traits::Service<T>::interface_name() method seems to be used for two distinct and orthogonal concepts in dbus-cpp:

 * in core::dbus::Service::add_service<T> and use_service<T>, it is used to determine a bus name associated with a type.

 * in core::dbus::Object::emit_signal, invoke_method_*, get_property, get_all_properties, etc, it is used to determine the interface name associated with a type.

While it is common for a service registered with a particular bus name to also respond to method calls that are part of an interface with the same name, it's not uncommon to implement methods from multiple interfaces.

Perhaps it would make sense to add a second bus_name() method for use by add_service and use_service?