Python-redux

- refactor: use `Immutable` from python-immutable package (extracted and created
based on `Immutable` class of this package)

- refactor: propagate new `FinishEvent` when `FinishAction` is dispatched

- feat: add `scheduler` option to schedule running actions in the main loop of frameworks
- feat: add `threads` option to run event handlers asynchronous in `threads`
number of threads
- refacotr: allow `Any` return type for event handler and subscriber functions
- feat: add `subscribe` property to the returned function of `autorun`

- feat: allow dispatching events with `dispatch` function

- refactor: postpone nested dispatches

- feat: drop `type` field in actions and events altogether, recognition is done
by `isinstance`