Handle branching in global states

[Aran30]

Currently, global states only allow linear conversations to take place.
In case of branching (e.g. 1 state with 2 different transitions), the global state will act arbitrarily.

Goal: Global states should support branching states.

[jcabot]

Global states should be an exception in the state machine, not the rule. So I wonder whether we really need to invest a lot on them . Users can always use normal states to simulate a glboal state with branching if they really need so