EnableNotifications not handling callbacks correctly

[TheRealDJ]

In the EnableNotifications functions, there is no check to see if the callback passed is already on the list. For example, if you do:

c.EnableNotifications(notification_callback)
c.EnableNotifications(notification_callback)

the notification_callback function will run twice for each notification.

The callback is also not removed from the list when nil is passed as the callback, the code just sets the attribute to [0x00, 0x00]. For example:

c.EnableNotifications(notification_callback)
c.EnableNotifications(nil)
c.EnableNotifications(notification_callback)

the notification_callback function will run twice for each notification.