Allow width control of nav bar buttons

Encode the width as part of the spec as a float. For instance:
"home[.5]" is a home button with half its normal width.

The UI will allow users to adjust the width of standard buttons and
spaces from .25 to 1.75, excluding the app shelf, which fills all
available space.

Change-Id: Icd2f498c164933d61f55d779b3e9be0afaba9c2d