Refactor and group change handler data into an object

[kitchoi]

In the notifiers for observers, we have these pieces of information:

traits/traits/observers/_trait_event_notifier.py

Lines 44 to 57 in 4145b41

	handler : callable(event)
	The user's handler to receive the change event.
	The event object is returned by the ``event_factory``.
	If the handler is an instance method, then a weak reference is
	created for the method. If the instance is garbage collected,
	the notifier will be muted.
	target : object
	An object for defining the context of the notifier.
	A weak reference is created for the target.
	If the target is garbage collected, the notifier will be muted.
	This target is typically an instance of ``HasTraits`` and will be
	seen by the user as the "owner" of the change handler.
	This is also used for distinguishing one notifier from another
	notifier wrapping the same handler.

traits/traits/observers/_trait_event_notifier.py

Lines 69 to 72 in 4145b41

	dispatcher : callable(handler, event)
	A callable for dispatching the handler, e.g. on a different
	thread or on a GUI event loop. ``event`` is the object
	created by the event factory.

The handler, target and dispatcher are defined by the user at the time when observe is called (observe being the future replacement of on_trait_change). In other words, they don't change throughout the lifetime of the "observe" action and only the user has a say in what they should be.

They can therefore be grouped together, making it easy for them to be passed around and compared.

This refactoring will not affect the user facing API.