theme: Add background functions for single buttons

With the existing background functions, single buttons can not be
styled separately - on the left side, the style of the left button
is picked, and the right button's style on the right side.

As theme authors may want to add rounded corners to button groups
as a whole, it makes sense to treat the case of a single button in
a group differently.

https://bugzilla.gnome.org/show_bug.cgi?id=635683