This lets the user cycle between past windows. The rules for when a
window gets added to the history is the same as when a window is
considered the last marked.

In fact, now all the last mark does is swap the current window with the
previous one in the history.

Swap master now swaps the master window with the prior master window if
swapMaster is called while the master window is focused.

Change the Marking to use XMonad's extensible state rather than
hand-rolling it myself. Allowed me to delete the XPlus monad.