Add do_behavior

[wobrschalek]

Hi,

Currently actions are supported for entry, exit, and transitions.

A very important use case are functions that are performed while being in a state, called do_behaviors in UML and invoke in SCXML.
They are typically run as thread, that is spinned up after RTC.

Maybe there's a workaround right now I didn't see.
Lmk.
Thanks,
Robert