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.
This is a pretty straight forward change to remove the global front
variable and instead use a couple of accessor functions.
This hopefully will make it easier to keep track and debug when
the front screen changes.
