Method used to validate posted form-values.

NOTE: To work with this method, the derived class needs to implement a method "getRequiredFields()", returning an array of field to validate. The array returned by getRequiredFields() has to fit the format [fieldname] = type, whereas type can be one of string, number, email, folder, systemid The array saved in $this->$arrValidationErrors return by this method is empty in case of no validation Errors, otherwise an array with the structure [nonvalidField] = text from objText is being created.