BDeskbar: Add a means to hide app from list of running apps

[humdinger]

This is hrev57308, 64bit.

For some applications it would be handy if their entry in Deskbar's list of running application could be hidden/unhidden at runtime. (For example, an application like BeScreenCapture doesn't want to appear in the shot when recording, but still appear when interacting with the user.)

ATM, all you can do is set the "Background app" flag in an app's resources, but that cannot be changed at runtime.