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.
* 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
