Add the ability to hide pinned tabs using the WebExtensions API

[aminought]

You've added tab grouping, but it's inconvenient to use them without workspaces. The only alternative is an extension like Simple Tab Groups, but the pinned tabs are shared between all the workspaces. There is an item in the documentation: "Tabs that are pinned cannot be hidden". I propose to reconsider this decision.

[Jon]

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