renamed some classes and Twig functions to more descriptive names (refs #6871)

HttpContentRenderer has been renamed to FragmentHandler.
The RendererStrategy subnamespace has been renamed to Fragment.
The strategy classes now have Fragment in their names.
ProxyRouterListener has been renamed to FragmentListener
The router_proxy configuration entry has been renamed to fragments.
merged branch fabpot/renderer-renaming (PR #6925)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #6925).

Commits
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0586643 renamed some classes and Twig functions to more descriptive names (refs #6871)

Discussion
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renamed some classes and Twig functions to more descriptive names (refs #6871)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6871
| License       | MIT
| Doc PR        | symfony/symfony-docs#2205

Todo:

 - [x] update the docs

In #6871, @kriswallsmith wondered if the names used for the ESI/HIncludes sub-framework were meaningful enough.

I agree that this was not the case and I propose to remove the notion of **sub-requests** in favor of **fragments**. This sub-framework is a way to render fragments of a resource (the fact that it's done via another request is an implementation detail).

With that decision, all names can be renamed and are probably more meaningful. Some examples:

* `HttpContentRenderer` -> `FragmentHandler`
* `RouterProxyListener` -> `FragmentListener`
* `router_proxy` -> `fragments` (configuration entry)
* `DefaultRenderingStrategy` -> `InlineFragmentRenderer`

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by fabpot at 2013-01-31T09:50:14Z

I forgot to mention that this renaming will also probably help documenting the feature as understanding the notion of a fragment is probably easier.

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by Tobion at 2013-01-31T14:18:40Z

I'd like to say again that the word `render` in `FragmentRenderingStrategy` does not fit because it's not about rendering (that's left to the templating) of a fragment.
I suggest to rename it to `FragmentInclusionStrategy` because it defines the strategy to include the fragment.
This is also consistent with every strategy there is:
- ESI: `<esi:include`
- SSI: `<!--#include file="header.shtml" -->`
- hinclude (even in the name)
- inline (similar to phps `include()`)

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by fabpot at 2013-01-31T14:48:07Z

I've just renamed `FragmentRenderer` to `FragmentHandler` and strategies like `EsiFragmentRenderingStrategy` to `EsiFragmentRenderer` (and everything is put into a new `Fragment` sub-namespace).

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by fabpot at 2013-01-31T21:11:44Z

I've reverted the Twig function name change as the current name is more expressive with its arguments: `render(controller(...))` or `render_esi(url(...))`.