Beaker: Hide tab-close button on background tabs (when there are many tabs)

[da2x]

Describe the Bug
Overcrowded tab bars have been a source of frustration ever since tabs were introduced into browsers. It’s easy to accidentally close background tabs when you try switching to them.

To Reproduce

1. Open many tabs.
2. Hover the mouse cursor over a background tab.

Expected Behavior
The close button shouldn't appear for background tabs when there are many tab; only show it on the current active tab. Both Firefox and Chrome has this behaviour for a reason.

[pfrazee]

I'm +1 this.

[pfrazee]

The easiest thing to do was to just always hide the non-active tab's close button so that's what I did here