Code review comment for lp://staging/~zsombi/ubuntu-ui-toolkit/bottomEdge

I find the line on top of the bottom edge when it is open ugly.

https://www.dropbox.com/s/mvexgb66nt5nr3n/Screenshot%202015-11-23%2021.27.24.png?dl=0

Also, see the comments I added to the API doc, and one diff comment below (more to follow).