Middle-clicking a tab in the list should close the tab

[Darthfett]

To mimic the behavior in the browser, middle clicking should close a tab.

[philiprhoades]

Hmm . . I would prefer to Right Click and to need a confirm . . otherwise it is too easy to lose something that is actually wanted kept.

[cxw42]

PRs welcome! If you do open a PR, please add a checkbox in Settings | Behaviour, default off, that controls the feature.

[cxw42]

#160 is another possible middle-button function, so the middle-button function should be a dropdown.