bug: Events triggered via sml::back::process<TEvent>() are not processed in parent state

[rhaschke]

Expected Behavior

Events not handled in a child state should be handled by the parent state. Duplicate of #400.

Actual Behavior

While this works for events injected via process_event() from the top-level machine, it fails for events triggered by an action via sml::back::process<TEvent>(): https://godbolt.org/z/dnMv1dfa4

[rhaschke]

I analyzed this issue a little bit and found that the event triggered via sml::back::processs is injected into the sub state's process queue instead of using the root state's one:

back::process	process
sml/include/boost/sml.hpp Line 1873 in 4be9dd6 return back::process<TEvents...>{sm.process_};	sml/include/boost/sml.hpp Line 2128 in 4be9dd6 aux::get<get_root_sm_t<TSubs>>(subs).process_.push(event);

A similar issue was resolved in #302 for process, which actually works correctly now.
I added a (failing) unit test in my fork to illustrate the problem.

To resolve that issue here in a similar fashion, one would need to access the root sm in get_arg, which is not available yet. However, instead of applying this kind of fix again, I want to fundamentally question the design:
Why do we need the process_ queue in all sub state machines at all? Doesn't it suffice to have a single one in the root state machine - as we need to process all events from there? All subs could simply have a pointer to that single queue instance, such that we don't need to change API of get_arg.

For deferred events the story is different I think: they should be kept in state-local queues and be processed as soon as the state is left.