Buttonset: Don't shift the last button to the left

This makes it much easier to style vertical groups of buttons and buttonsets properly.

NOTE that this will only work so long as the last .ui-button in the buttonset is also the last child of the parent.

Example
Suppose you have three buttons and one buttonset in a column, and want their edges to align nicely:

|          Button1           |
|          Button2           |
| Radio1 || Radio2 || Radio3 |
|          Button3           |

Before, this was near impossible since the buttonset would always be offset by a bit on one side or the other. By not applying the shift-right style rule to the last button in the buttonset, however, no special margins poke out of the sides, and styling it all becomes much easier.