Allow calling processEvents() from listener callbacks

[nsk90]

Currently such pending events are passed to PendingEventHandler and there are three options:

• to throw exception in this case (default)
• ignore such events
• store them in user managed queue (user is responsible for calling processEvent for them when current processing completes)

Such queue may be implemented on library side allowing processing of pending events by default.

[nsk90]

Done in https://github.com/nsk90/kstatemachine/releases/tag/v0.13.0