Comment 7 for bug 336110

The current bubble width is 18 ems. Mat and I have just experimented with mockups of bubbles of different widths, and at any width beyond 20 ems the bubbles started looking weird, especially those for volume/brightness. So, I've updated the spec to say that bubbles should be 20 ems wide. <https://wiki.ubuntu.com/NotifyOSD?action=diff&rev2=155&rev1=154>