Finds, loads and unloads plugins.
Returns a list of available plugins.
Load classnames from file and construct modulename relative to plugindir from plugindir, filename and file entries.
Returns a dictionary: classname -> module.
Loads the given plugin and unloads possibly sooner loaded plugins.
Test if given module contains a valid plugin instance.
Returns None if not or (name, modulename) otherwise.
Unload currently loaded plugin.
Import the module and return modname.classname.
Module author: Bastian Venthur <venthur@cs.tu-berlin.de>