This allows a keyboard shortcut to move a client to a different amiwm
screen. That way when you have things like Firefox restart every window
in a single screen, you can quickly move screens to where they should be.
This adds a command to iterate over the set of windows. It's not
like alt-tab in ye olde windows world where you'd alt tab once to
switch between two windows, and then keep alt-tab'ing with alt down
to go through the window list. For now it just literally iterates
through those windows.
This makes it a lot easier to work with a large number of open
windows without having to constantly use the mouse.
