Display tabs with color from Containers Test Pilot

[Fabiensk]

Containers is a nice experimental add-on from Mozilla allowing to segregating tabs by context (called "container" ; ex: I don't want my shopping tabs to communicate cookies from other tabs). When a tab is open for a given container, it will be displayed with the color of the container. It is so for the classic tabs, but also for the "Tab Center" add-on. It would be nice to have such distinctive display in Tab Tree.

Some research:

• Code for Containers: https://github.com/mozilla/testpilot-containers
• Code for Tab Center: https://github.com/bwinton/TabCenter/
• The color for a container is assigned using a CSS variable. I don't know how it would be possible to retrieve this attribute in Tab Tree. Maybe Tab Tree and Tab Center are architectured diffently, making it difficult to get the info.