ENH: Specification of foreground, background, and barrier classes.

Fg and Bg are two competing object classes, while barrier will
be classified as NotObject.

Change-Id: Ic7c7e93f3d094e838d6a07114dfceacd9e915636
Merge topic 'add-background-foreground-barrier-selectors'

0de2c75 ENH: Specification of foreground, background, and barrier classes.