Localized label in validation rules

[michalsn]

Description
This pull request allows the easy localization of labels in form validation rules.

Using a "dot notation" for a label in validation rules, like: Foo.bar will search for file en/Foo.php and line bar. If the file or line doesn't exist the usual text will be returned.

Ref: #2845

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide