Centering tab icon and close icon (by removing unnecessary right space)

`margin-left: 3px` create `6px` left of the icon, the width that produce a centered icon is then `3 + 3 + 16 + 3 + 3 = 28`.

The close button width is 8px and margin 7px, the width to produce a centered close icon is then `3 + 3 + 16 + (3 + 7) + 8 + (7 + 3) + 1 = 51`.