Fix StartWindozeCycle() shift key behaviour.

Fix StartWindozeCycle() shift key behaviour.

As the name implies, StartWindozeCycle() cycles windows in the same way
that a popular commercially-available operating system does.  However
Window Maker's handling of the shift key in the switchpanel does not
currently mirror that of its commercial counterpart.

In the popular operating system:

  Holding alt and shift then pressing and releasing tab will highlight
  the previous window in the switcher.

  Releasing shift with alt still held will not close the switcher.

  The window change is commited when alt is released.

In Window Maker:

  Holding alt and shift then pressing and releasing tab will highlight
  the previous window in the switchpanel.

  Releasing shift with alt still held will close the switchpanel and commit
  the window change.

This patch adds the StrictWindozeCycle boolean preference.  When it is
set to YES the switchpanel will remain open as long as alt is held even
if shift is pressed and released.