using computed width instead of content width to give users flexibility to change box-sizing

[2is10]

This commit does not change anything for most users -- those who do not specify box-sizing for the token-list ul or who specify box-sizing: content-box. It just gives users the flexibility to add left/right padding to the token list ul and set box-sizing: border-box on it and the dropdown, so that their widths will still match, despite the padding.

Note that .css("width") is also a performance win over .width().