Close tab on middle-click

Also make sure we only fire regular commands when left-click is used,
not for any buttons
Shorten function name to enableTabMiddleClick
Merge pull request #107 from elisee/middleclick-closes-tab

Close tab on middle-click (fixes #106)
Make tabs grow up to 200px by default
Resize tabs only when mouse leaves tab bar

Enforce a consistent tab size and add a temporary ghost tab to the end
of the tab bar when a tab is closed, to prevent tabs from resizing
immediately.

This behavior, inspired by Chrome, makes closing a bunch of tabs in
quick succession reliable and predictable. Ideally we'd want the tabs
resizing to animate, but this is already a worthwhile improvement as it
stands.