Allow ajax response short-circuit/override

Currently, JSF handles an ajax response, including the re-rendering of the
specified components, every time, even if onevent is specified. There are times
when the ajax response requires custom handling. For example, the HTML returned
after rerendering a large data table might need to be passed to a Javascript
widget so it can update its state in a widget-specific manner. There is
currently no way to do this.

I would like to see the ajax portion of the spec modified to allow client code
to override the response handling, thus assuming the full burden of updating the
client.
#### Environment
Operating System: All
Platform: All
#### Affected Versions
[2.0]

No implementation file or test is identified in the issue. Start by reviewing the JSF ajax portion of the specification and the existing client-side response handling, then define how client code overrides rerendering and when the feature is complete.

Written by the indexing model from the issue text.