Move activeWebExtensionTabId from WebExtensionsMiddleware to WebExtensionState

[pocmo]

#7867 will introduce a Middleware that will take care of calling markActiveForWebExtensions() on EngineSession instances. For doing that it internally remembers which is the current active tab id. This state could be moved to WebExtensionState (after splitting it into global and session-local state). This avoids having additional state inside the Middleware. ALL STATE TO THE STORE.

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