StackView: Clean up the defunct preferredWidth property.
StackView: Clean up the defunct preferredWidth property.

It was a holdover from the previous nesting design.

osric-the-knight/foam,mdittmer/foam,foam-framework/foam,mdittmer/foam,jacksonic/foam,foam-framework/foam,foam-framework/foam,mdittmer/foam,jacksonic/foam,jlhughes/foam,jlhughes/foam,osric-the-knight/foam,osric-the-knight/foam,jacksonic/foam,jlhughes/foam,jlhughes/foam,foam-framework/foam,jacksonic/foam,foam-framework/foam,osric-the-knight/foam,mdittmer/foam