Support XBEL

[GreenLunar]

Following comment by @Dalethium.

I think, if this were to be integrated, it may be more fruitful to use a more widely adopted external bookmark system such as XBEL -- that way more widely supported systems such as Flocus could be used along with many others.

I think that using something like XBEL is smarter since it's a format standard, and not a specific project that Yacy would have to then integrate and work with. That way, if other projects wanted to integrate with Yacy's bookmark system, they could follow a standard format and not depend on this project or any other project for integration practices.

https://floccus.org/assets/img/screen_select_sync_method.png