The plugin refcounter is driven from interface opening/closing, this is
quite internal.
The 'last' time in plugin now records the last time the refcounter dropped
to 0, this is just sufficient enough to find out how long a plugin is
unused.
* add a lumiera_interface_version() function for runtime type and version
checking of interfaces
* the 'lumieraorg__plugin' interface is now private
* add a macro to construct a interface instance name string
* 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.
