icingaweb2-module-incubator base implementation for HTML forms is susceptible to CSRF

icingaweb2-module-incubator is a working project of bleeding edge Icinga Web 2 libraries. In affected versions the class `gipfl\Web\Form` is the base for various concrete form implementations [1] and provides protection against cross site request forgery (CSRF) by default. This is done by automatically adding an element with a CSRF token to any form, unless explicitly disabled, but even if enabled, the CSRF token (sent during a client's submission of a form relying on it) is not validated. This enables attackers to perform changes on behalf of a user which, unknowingly, interacts with a prepared link or website. The version 0.22.0 is available to remedy this issue. Users are advised to upgrade. There are no known workarounds for this vulnerability.