Revision 631

It was decided that if the user closes
a dockable window the window should simply hide itself. That
way if the user wants to show the dock again it appears in the
same place as it was when it was closed.
(DockableWindow.Destroy): Call renamed method OnDockDestroy().
(DockableWindow._OnButtonClose): Hide the window instead of
        destroying it.
(DockableWindow._OnClose): Hide the window instead of destroying it.