Offer various layouts for the Gallery macro

See https://forum.xwiki.org/t/improvements-for-images-in-xwiki/9170

Important info mentioned:

For image grouping we already provide a `gallery` macro. That’s good. However we could offer a `layout` parameter to let the user decide how to lay out the images. The current default would be `slideshow` but we could add `tiled` and others, [like in MediaWiki ](https://www.mediawiki.org/wiki/Help:Images#Mode_parameter).

I see we have a `class` parameter but I tried to use it (as in the example on the doc page) but it didn’t do anything. Also I think it’s not user-friendly enough and I think the concept of `layout` would be simpler to understand for users.

And

After discussion with Marius we said:

• It’s good to extend the gallery macro.
• The macro is currently using its own JS for the image display and with the introduction of different layouts, it would be better to explore existing JS libraries and to change the gallery macro implementation to use it.
• An example of such a library is FancyBox (Note: a [fancybox macro has been contributed ](https://extensions.xwiki.org/xwiki/bin/view/Extension/Fancybox%20Macro/)).