Each window now has an associated progress integer that can be updated
via the SetWindowProgress IPC call.
This can be used by clients to indicate the progress of ongoing tasks.
Any number in the range 0 through 100 indicate a progress percentage.
Any other number means "no progress"
This allows windows/widgets to learn when something is being dragged
over them. They can then repaint themselves somehow to indicate that
they are willing to accept a drop.
Currently this is piggybacking somewhat on the mouse event mechanism
in WindowServer. I'm not sure that's the best design but it seemed
easier to do it this way right now.
