Tab bar behavior improvements

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.

I also re-enabled but shortened the tab width opening animation (was .5s, made it .2s).

(Also: yay, I managed to rebase my commits properly after screwing up the first time around! GG me)