monitoring: optimize imports in the backend configuration form
monitoring: Use '@return $this' when documenting fluent interfaces in the backend configuration form
monitoring: add description to @see tags in the backend configuration form
monitoring: use @type instead of @var in the backend configuration form
monitoring: Fix PHPDoc of BackendConfigForm::setResourceConfig()
monitoring: Fix missing return argument in the backend configuration form
monitoring: Uppercasse the first word of comments in the backend configuration form
monitoring: Use Form::translate() in the backend configuration form

refs #7551
Fix module domain detection in Form::getTranslationDomain()

The usage of preg_replace had two errors:
1) The regular expression was wrong
2) $matches[0] always contains the full matched string, not the first parenthesized subpattern

The correct version of preg_replace would've been:

if (preg_match('/^Icinga\\\\Module\\\\([A-Za-z]+)\\\\/', get_called_class(), $matches) === 1) {
    return strtolower($matches[1]);
}

But since there's no benefit of using a regular expression here except less speed, I replaced it
with using explode.

refs #7551