Add pre and post ajax transaction function callbacks

Use case: say the user wants to use some effect to fade in/out the content to be updated via Ajax. It
would be nice to have a way to install pre-request and post request callback functions. On the post
request, we need to have actually two functions. One when we have the response but have not yet done
the replacement, and another after we have done the replacement.
#### Environment
Operating System: All
Platform: All
#### Affected Versions
[2.0]

Start by reviewing the Jakarta Faces Ajax transaction lifecycle and any existing callback mechanisms. Define callbacks for before the request, after the response but before replacement, and after replacement; done means all three points can be used for effects such as fading updated content.

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