[Merge] lp:~osomon/webbrowser-app/close-tab-middle-click-X into lp:webbrowser-app

[Ugo Riboni]

I merged this change as part of https://code.launchpad.net/~uriboni/webbrowser-app/larger-close-tab-button/+merge/272557
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https://code.launchpad.net/~osomon/webbrowser-app/close-tab-middle-click-X/+merge/272699
Your team Ubuntu Phablet Team is requested to review the proposed merge of lp:~osomon/webbrowser-app/close-tab-middle-click-X into lp:webbrowser-app.