This ditches the old 'proof of concept' implementation, interfaces and
plugins are internally separate now and publically only accessed over
the 'interfaces' api.
The interface registry now gets an additional 'plugin' member to
backreference plugins from interfaces.
* lumiera_interface are now static structures, never wrritten
* introduced a lumiera_interfacenode which manages the dynamic data
of interfaces. The interfaceregistry now holds this nodes.
