Potential memory leak submitted from a customer.

This class (in an effort to self-reportedly enforce MDI
single-selection model) inadvertantly holds on to a strong reference to the
currentFrame.

The WindowsDesktopManager overrides the DefaultDesktopManager
activateFrame(JInternalFrame) method, but does not override the
deactivateFrame(JInternalFrame) or closeFrame(JInternalFrame)
methods to unassociate that strong reference.

Also, the package private JInternalFrame initialFrame is no longer used
and can probably be removed.