Update Manager forgets user's selection if settings window is opened

[Mark]

This issue applies to update-manager in both 11.10 and 12.04.

Steps to reproduce:
 - Start Update Manager
 - Spend several minutes reading descriptions of the various updates, selecting some and de-selecting others.
 - Click the Settings... button at the bottom of the window
 - Without changing any settings, click Cancel to close the settings window.
 - Update Manager forgets which packages you selected before and instead reloads the list of updates (maxing out CPU for several seconds), with all updates selected.

Update Manager does not need to reload/refresh the list of updates if no settings have changed.

Even if the user did change some settings, it could remember which updates were selected/unselected, and after reloading the list, make the initial selection state match what the user chose before. (As opposed to defaulting to having all updates checked.)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.