Various changes regarding the page forms

This pull request makes various changes regarding the page forms:
- Removed PackageID from `\wcf\system\language\I18nHandler::save()` calls since that’s already the default value: https://github.com/WoltLab/WCF/blob/master/wcfsetup/install/files/lib/system/language/I18nHandler.class.php#L258
- Outsourced validation of aliases to a new util-class. Moreover, a issue when validating the alias against the pattern was fixed.
- Restructured the `validate` method of the page forms. The validation of alias and menu item is outsourced to individual methods. Thereby the edit form don't have to overwrite the complete validation method but only the two sub-methods.
- A foreign key is used to store the parent page in database. Notice: `parentID` for pages without parent is now `null` (prior `0`)
- Replaced the awkward error message regarding an empty alias (just `Enter Alias!`) with the standard error message for empty but required form inputs.

Though I`ve tasted all my changes, you should test everything yourself before merging since this pull request contains one far-reaching change (changes regarding the foreign key).