UX: add description as tooltip

Grimreaper → credited vmaucorps → .

Discussed with @pdureau, currently no link to the library.

Add style's description to select list as description. But provide it as a tooltip.

As there is no tooltip API in core (at least for the moment, or I missed something), need to make a custom implementation like Webform did.

I will prepare a MR to add the description.

And also a MR for Gin, to be able to have it working at least in custom projects.

Let's wait for core to provide the Tooltip feature.