Improve tab managment by giving windows a custom sidebar title

Oftentimes, people tend to group tabs thematically in different windows. It would be nice to give those windows a custom title. The title could be displayed as the headline of the sidebar.

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/sessions/setWindowValue
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/sessions/getWindowValue