Slow loading time when changing the icons theme to Silk in the Icon picker

Note that a similar performance issue has affected the icon quick action in the WYSIWYG editor which lead to XWIKI-22537. I fixed the Icon Service used by the icon quick action to have better caching and perform icon filtering server-side. This doesn't fix the Icon Picker (this issue) though. In the case of the icon quick action the fact that the request to fetch all icons is slow is not a blocker, because users are usually typing to filter the icons. For the Icon Picker we may need some kind of pagination.