Support observing Camera events

[Sija]

Like below:

camera.on :file_added do |event|
  # ...
end

[Sija]

I guess it would have to be sth along the lines of:

# Implemented somewhere inside of `Camera`
spawn do
  loop do
    event = wait timeout: 2000 # default, 2 sec
    event_name = event.type.to_s.underscore
    notify_listeners event_name, event
  end
end

Important questions/issues along the way:

• is it needed? - YES, would come handy.
• is it doable?
• will it block the camera?
• how big should be the timeout value?

[Sija]

https://github.com/krisleech/evented looks good for the job.