Fix Popover zindex UI issue

[stephane-klein]

Now that table header row css is position: sticky;, I have this UI issue :

The goal of this issue is to fix this bug.

[stephane-klein]

Ressources:

• https://popper.js.org/docs/v2/modifiers/flip/
• https://github.com/bryanmylee/svelte-popperjs

[stephane-klein]

Corrigé par https://github.com/stephane-klein/poc-sveltekit-headless-table/commit/65fc854fd30b88d6284b225383ad3ca1f581bcfa

[stephane-klein]

En tout, j'ai passé 6h sur cette issue ! (j'inclus dedans le temps passé sur https://github.com/stephane-klein/backlog/issues/351)