intermdiate commit. Added more structure to support unittests.

Will still have to look through the plugin code, and change it, probably write tests right away
Very intermediate implementation of a slimmed down plugin.

However, I think the backlog functionality can't properly work if a single plugin
has multiple callbacks, as all this capability would have to be stored per callback, but is
only stored per plugin.

Therefore, each plugin can only have a single (failable) call, which shall be abstract as well
Removed support for multiple callbacks per plugin