Bug 1042960 - Create UI elements for facet bar

This is a WIP that addresses two challenges with the design for bug 1042960.

1) One-sided borders are tough. They require hacky layer lists or a 9 patch, which is doable, except:
2) How would we change their colors based on the search engine? 
If we replaced BottomBorderTextView with a less general class (FacetButton), perhaps some of this boilerplate (eg "clickable") could be encapsulated?