convert response callbacks into a tween

[mmerickel]

There is a backward incompatibility here in how tweens are defined. This
is consistent with the new view deriver API but may present some
frustration for anyone currently defining a tween under the EXCVIEW
as all tweens now default to over the EXCVIEW.

This incompatibility is not strictly required to implement this feature,
but if we don't then:

1. Tween ordering is almost completely dependent on the user's
   environment and sorting behavior. If they add another tween then the
   addon tween (if left unspecified) may be shifted to a bad place.
   This is an issue unrelated to this feature... just a lingering issue with
   tweens.

2. Another incompatibility may surface in which a user's tween would
   show up OVER the response callbacks tween, which would lead to
   inconsistent behavior versus the previous pipeline where callbacks
   were always executed after tweens.

This patch shifts tweens to be more fully specified which should be
generally considered a good thing.

Side note, the response callbacks and excview tweens are defined
with fallbacks incase one of them is overridden or unavailable for some
reason by using a list of dependencies. We should apply this pattern to
the view derivers as well to make them easier to override.

Closes #2622.

[digitalresistor]

LGTM

[mmerickel]

I'm working on docs for this now.

[mmerickel]

There's another issue with this feature that I just noticed.

The default for request.invoke_subrequest is use_tweens=False. Previously the response callbacks would have been executed here... this changes things so that callbacks are not executed. This incompatibility concerns me.

[mmerickel]

Updated the docs but I'm worried about the subreqest situation. It feels like a fairly large incompatibility. I argued when the feature was introduced to keep use_tweens=True and honestly use_tweens as an option seems like a bad idea in general (but it's there for things like the transaction manager that cannot be nested when using the threadlocal manager... which we're starting to move away from) but it's there now so we have to deal with it.

[stevepiercy]

I'll review docs when the implementation is finalized. Please ping me at that time, and I'll dive in.

[mmerickel]

I'm going to close this and port over only the changes from config.add_tween that explicitly add tweens OVER excview unless otherwise specified.