[Merge] lp:~uriboni/webbrowser-app/undo-close-tab into lp:webbrowser-app

[Olivier Tilloy]

Review: Needs Fixing

Incognito tabs are incorrectly being remembered: launch the browser, switch to private browsing, browse to a given page, close it with Ctrl+W, this switches back to public browsing, then press Ctrl+Shift+T: the private tab you just closed is being restored, as a public one.
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https://code.launchpad.net/~uriboni/webbrowser-app/undo-close-tab/+merge/275849
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