[Serializer] Added XML attributes support for DomDocument in XmlEncoder.

This is a combination of 2 commits.

- [Serializer] Added encoding support for DomDocument in XmlEncoder

- [Serializer] Refactor code to allow setting <?xml standalone ?>

This commit refactors the createDomDocument(..) method in XmlEncoder
so
it can set the 'version', 'encoding' and 'standalone' attributes on
the
DOM document.

Code coverage of new code: 100%. Tests: pass.
[Validator] fixed StaticMethodLoader trying to invoke methods of abstract classes (closes #8589)
removed dead code and fixed CS
merged branch fabpot/validator-abstract-fix (PR #8592)

This PR was merged into the 2.2 branch.

Discussion
----------

[Validator] Fixed StaticMethodLoader does not try to invoke methods of abstract classes anymore

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8589
| License       | MIT
| Doc PR        |  n/a

Commits
-------

b41cf82 [Validator] fixed StaticMethodLoader trying to invoke methods of abstract classes (closes #8589)
merged branch fabpot/dead-code (PR #8572)

This PR was merged into the 2.2 branch.

Discussion
----------

removed dead code and fixed CS

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

7d58147 removed dead code and fixed CS
merged branch fabpot/console-fix (PR #8600)

This PR was merged into the 2.3 branch.

Discussion
----------

[FrameworkBundle] fixed regression where the command might have the wrong container if the application is reused several times

Commits
-------

1bd45b3 [FrameworkBundle] fixed regression where the command might have the wrong container if the application is reused several times
[routing] added ability for apache matcher to handle array values
[HttpFoundation] deprecated FlashBag::getIterator() method
[Console] made descriptors use output instead of returning a string

[console] made descriptors use output instead of returning a string
[console] updated descriptors usage
[console] fixed descriptors usage & tests
[console] applied advices from github
[Console] applied advices from github

updated changelog
merged branch jfsimon/issue-8371 (PR #8451)

This PR was merged into the master branch.

Discussion
----------

[console] makes descriptors use output instead of returning a string

This PR changes `DescriptorInterface::describe($object, array $options = array())` method signature to `describe(OutputInterface $output, $object, array $options = array())` and all subsequent implementations.

It's now possible to use helpers inside descriptors.
That was not easy!

@lyrixx this is all for you
@fabpot I just added one new class

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8371

Commits
-------

b9fa52c [Console] made descriptors use output instead of returning a string
[WebProfilerBundle] made toolbar listener instantiation conditional
[Console] fixed tests
Revert "[Form] Remove "value" attribute on empty_value option"

This reverts commit 9e849eb78bcc668a2b8aad39cb47e2ad32c73a47.

Reasons for the revert:

* https://github.com/symfony/symfony/issues/8478
* https://github.com/symfony/symfony/issues/8526
[Form] Removed the "disabled" attribute from the placeholder option in select fields due to problems with the BlackBerry 10 browser
Fixes link indices
[Form] Changed (Number|Integer)ToLocalizedStringTransformer::reverseTransform() to do rounding
[Form] Fixed: If a form is not present in a request, it is not automatically submitted
Revert "[Form] Fix of "PATCH'ed forms are never valid""

This reverts commit a2b15359d81563d52b62b4357407efa949a0706a.

Conflicts:
	src/Symfony/Component/Form/Form.php

The commit is reverted because it introduces a bug demonstrated by a currently failing test.
[Form] Fixed patched forms to be valid even if children are not submitted
Fix issue with \DateTimeZone::UTC / 'UTC' for PHP 5.4