• BaseEvent isn't serializable, so I replaced it with a dict internally and convert that to a BaseEvent only when passing it to the extension class (need to do this for backward compatibility).
• Also converted ExtensionCustomAction from a class to a method creating an event event:activate_custom directly.
• Changed the logging to no longer log the whole object, but instead it now converts the dict to a set and logs that as the "keys of" the object.

Had to rush it a bit as I discovered v6 branch was broken and this PR fixes it (still prefer to call it refactoring though). So I deleted the test_Extension.py test as I didn't have time / understand how to fix it. Also think importing DeferredResultRenderer and ExtensionServer into UlauncherWindow is not optimal, but it will have to do for now.