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

library/Icinga/Web/Form.php

@@ -806,19 +806,20 @@ class Form extends Zend_Form
     }
 
     /**
-     * Return the translation domain for this form
+     * Get the translation domain for this form
      *
-     * The returned translation domain is either determined based on
-     * this form's class path or it is the default `icinga' domain
+     * The returned translation domain is either determined based on this form's qualified name or it is the default
+     * 'icinga' domain
      *
-     * @return  string
+     * @return string
      */
     protected function getTranslationDomain()
     {
-        if (preg_match('@^Icinga\\\\Module\\\\([A-z]+)\\\\.*$@', get_called_class(), $matches) === 1) {
-            return strtolower($matches[0]);
+        $parts = explode('\\', get_called_class());
+        if ($parts[1] === 'Module') {
+            // Assume format Icinga\Module\ModuleName\Forms\...
+            return strtolower($parts[2]);
         }
-
         return 'icinga';
     }