Add method for synchronizing to the UI

[mr-highball]

A request was made in #14 to simplify synchronizing events to the UI.

AC's

• add a method to queue a callback for UI
• should provide the reference to this ezthread

Notes

• could we leverage a new ezthread and use the synchronize on stop event?

[mr-highball]

Not sure why that method uses a global critical rather than a member one (this is probably the answer to avoid deadlocks)