Add dialog to MaxGUI displaying the free/enterprise information

MXS-5499 will make free/enterprise information available via the REST-API. Whether the version used is the free or enterprise one, should be made visible somehow. The free version should further have in an About dialog a link to a MariaDB page that acts as an entry-point to the enterprise version. The URL should be provided by MXS-5499 and not hardcoded into MaxGUI itself.