Feature #4351: Allow displaying deleted values

Make settings browser support a little checkbox to enable/disable the display of uninstalled products, or the display of deleted values. Remember the checkbox setting using the settings engine API. Add a bit of infrastructure so that the settings browser can easily support storing other settings as well, and so that it notices when these settings change on another machine, and reflects that appropriately in the UI.

Fix a silly little logging copy+paste bug in the engine as well (backgrnd.cpp) that can cause a crash.