Popover API migration

[jarek-foksa]

<x-popover>, <x-notification> and <x-menu> widgets should be migrated to the new Popover API. Currently this API is available only in Chromium >= 114 and Safari 17 (Developer Preview), so I will have to wait at least until early 2024 before making the switch.

Documentation:

• https://developer.chrome.com/blog/introducing-popover-api/
• https://developer.mozilla.org/en-US/docs/Web/API/Popover_API