Merge lp://staging/~dylanmccall/update-manager/refactored-cellareapackage into lp://staging/update-manager

While playing with the package column (and pkg_area) in UpdatesAvailable.py, I noticed I was needing to perform shotgun surgery in order to make some small changes: there was a bunch of code in CellAreaPackage that depended on specifics of UpdatesAvailable's tree view. This branch contains a more generalized CellAreaPackage. It isn't exactly _portable_ at this stage, but I think it's a step in the right direction. The assumptions it continues to make are documented, and innocent changes in other parts of the program are less likely to break it. The output should be identical to the existing cell area.