* fix configure.ac to check for dlopen, introduce $(LUMIERA_PLUGIN_LIBS)
* Generic pluginloader finished, all plugins get loaded and registered at
start for now, it is prepared for on-demand loading and unloading but
not implemented because that needs the plugindb.
- plugin unloading not yet finished, this asserts at the end
* plugin{.h,.c} are back but work in progress
* the interfaceregistry also manages a pluginregistry, makes no much sense
to isolate it yet, both share a mutex and are commonly used together
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.
