Handle Multi-Account Containers extension

[Korko]

Mozilla Firefox have an official extension called Multi-Account Containers allowing to have "categories" of tabs each having their separate cookies and so possibly a different account on each. https://github.com/mozilla/multi-account-containers/#readme

By default, YouTubePopupPlayer opens a tab without a specific container. It may be very interesting to keep the container of the original tab if possible.

[rthaut]

This is a great idea for an enhancement; thank you for the recommendation.

I know that a different container extension was causing the popout to be closed immediately (see #64), so any container-related changes should probably go through some extensive testing.

Here are my notes (for myself or someone else) for implementation ideas based on my initial investigation:

The documentation shows it is possible to open a new tab using a specific contextual identity (a.k.a. container), and it also appears to be possible to retrieve the contextual identity of the original/current tab.

The only downside to this approach is the "cookies" permission is needed to get/set the cookieStoreId property on tabs. Ideally this could be an optional permission for the Firefox build that is only requested when users "opt in" to enabling container support (likely via a new FF-only toggle switch on the Options panel), as I think many users would be apprehensive of using/updating the extension if the "cookies" permission was always required.

Alternatively, there might be a different way to open the popout so that the container extension(s) intercept the popout and handle it directly (like they do when clicking links on webpages within a container). However, based on the multitude of issues I ran into with other opening approaches during the version 3 rewrite, I'm fairly certain this won't be possible.