core (XMLUI): new XHTMLBox widget: XHTMLBox is a textbox specialised in XHTML, i.e. it renders the XHTML when in read_only, and it allows to edit it. The XHTML is cleaned by default, the cleaning is done by Text Syntaxes plugin (actually there is a cleaning method which can be set by any plugin, but Text Syntaxes is the one which does it).