Don't left-shift the last button in a buttonset

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.