removes the difference in compiled in and plugin interfaces.
* All interfaces are now defined with LUMIERA_EXPORT(...)
* The definition of LUMIERA_PLUGIN at compile time takes action to compile
as plugin which gets automatically managed.
* compiled in (core) interfaces need still be registered, this is
simplified with the LUMIERA_INTERFACE_REGISTEREXPORTED and
LUMIERA_INTERFACE_UNREGISTEREXPORTED macros which become no-ops for
plugins.
