Support Genshi MsgDirective
Genshi has this MsgDirective [1] which marks the content as translatable. This is useful for HTML with for example <a/> tag (or style).
For now it does not work in trytond because we do not add the Translator directives.
[1] https://genshi.edgewall.org/wiki/ApiDocs/genshi.filters.i18n#genshi.filters.i18n:MsgDirective:extract