[WCM] Skip console commands from event listeners

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#9235)
| Applies to    | master
| Fixed tickets | none
[Translation] added method to expose collected message
[OptionsResolver] Adjusted the OptionsResolver documentation to describe the 2.6 API
Added missing mailer class and use statements
Removed duplicate use statements
Adder and remover sidenote

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none

There are cases when you only want either adder or remover, important to know both of them need to be found if any is to work.
Merging emphasized notes

Merged both notes into one, in accordance to discussion feedback.
intro the var-dumper component
review effect
Reduce line length
doc for Dumpers
split intro/advanced sections
Merge pull request #1 from peterrehm/patch-9

Added mailer class and use statements
validate `null` (Expression constraint in 2.6)

Since Symfony 2.6, the Expression constraint doesn't skip validating
`null` values.
use VarCloner instead of Php/ExtCloner
first import of the "Official Best Practices" book
Fixed some technical problems as suggested by Wouter
Added an index directive to the first chapter
updated fabpot/sphinx-php module reference
added the missing "best-practice" directive import in Sphinx configuration