|This class implements a DesktopManager which more closely follows
the MDI model than the DefaultDesktopManager. Unlike the
DefaultDesktopManager policy, MDI requires that the selected
and activated child frames are the same, and that that frame
always be the top-most window.
The maximized state is managed by the DesktopManager with MDI,
instead of just being a property of the individual child frame.
This means that if the currently selected window is maximized
and another window is selected, that new window will be maximized.