Introduce a generic API to add tabs to the ElementBrowser and the RTE Link Wizard. A tab registry should manage the available tabs. Each tab has an associated TabHandler. Multiple Tabs might have tabs and should select the same TabHandler. right one for display depending on some parameters. (e.g. "act") A tab has: * an identifier * a title * a handler Optional followup: Convert the existing tabs to the new API, which gives better separation of concerns.