Add alpha UI for background cp section

This patch exposes to the UI control over the alpha level of the
background image.  It supports levels of 20% 40% 60% 80% and 100%
(utilizing the icons from the network view).  Because there is no 25%
icon, I made 20% be the default level.
Merge pull request #37 from manuq/background2

Add alpha UI for background cp section