[MonologBridge] Ignore empty context/extra by default in the ConsoleFormatter
[ExpressionLanguage] added ExpressionFunction and ExpressionFunctionProviderInterface
[DependencyInjection] added support for custom expression language functions
[Security] added support for custom expression language functions
[Routing] added support for custom expression language functions
[FrameworkBundle] added a compiler pass for expression language providers
[Form] Added "label_format" option
[Validator] Made it possible to store the cause of a constraint violation
[Intl] Integrated ICU data into Intl component
[Debug] add some file link format handling
[Debug] expose object_handle
[SecurityBundle] fixed tests when used in standalone
Merge branch '2.3' into 2.4

* 2.3:
  [SecurityBundle] fixed tests when used in standalone
fixed some composer.json to make standalone component tests pass
fixed standalone component tests depending on Validator and Form
[Validator] fixed component standalone tests
feature #12006 Expression language extensibility (fabpot)

This PR was merged into the 2.6-dev branch.

Discussion
----------

Expression language extensibility

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10512
| License       | MIT
| Doc PR        | not yet

The way we can add functions to an ExpressionLanguage instance is by using inheritance. #10512 tries to make the expression language in the routing flexible but using inheritance won't work when several bundles want to add functions.

So, this PR takes another approach to solve the problem globally.

Todo:

 * [x] add some more tests
 * [ ] add some docs

Commits
-------

7c24188 [FrameworkBundle] added a compiler pass for expression language providers
4195a91 [Routing] added support for custom expression language functions
1a39046 [Security] added support for custom expression language functions
79bcd52b [DependencyInjection] added support for custom expression language functions
184742c [ExpressionLanguage] added ExpressionFunction and ExpressionFunctionProviderInterface
fixed standalone tests
fixed standalone tests
[Debug] fixed class lookup when using PSR-0 with a target dir