Update `<post-menu>` and `<post-popovecontainer>` to enforce separation of concerns

[myrta2302]

Based on #6042.

• Enforce separation of concerns between the post-menu and the post-popovercontainer:
  → The post-popovercontainer should know about its own state and emit events when it changes.
  → The post-menu should only react to the events.
  → There should be an extra payload {first: boolean} in the post-popovercontainer events.
  → Instead of having a requestAnimationFrame in the post-menu, there should be a postToggled event in the post-popovercontainer

  The post-popovercontainer events would then be:

  • postShow Before content becomes visible (animation not started yet).
  • postShown After content is fully visible (animation ended).
  • postHide Before content starts hiding (animation not started yet).
  • postHidden After content is fully hidden (animation ended).
  • postToggle Before visibility is toggled (animation not started yet).
  • postToggled After visibility toggle is complete (animation ended).

And all these event would have a payload of type {first: boolean}.

• addEventListener must be avoided whenever possible
  → The post-menu should react to the postRendered event with a onPostRendered update in the renderer
  → Same for the onPostToggle or onPostToggled