Stop propagation makes events non-reusable

[hartym]

Although we may not want to change the current behaviour that can make sense in some cases, there are cases where the same event instance may need to be dispatched more than once, and it may be necessary to stop propagation in an intermediate listener.

Maybe add a reuse=True to dispatch/adispatch ? or ... ??

[hartym]

For 2.x this is addressed with a non-breaking, opt-in Event.reset() that clears propagation_stopped so the same instance can be dispatched again. PR #23.

Resetting propagation at the start of every dispatch (reusable by default) is a behaviour change, so it is tracked separately for 3.0 in #24.