Better MUI compatibility for Window class:

Better MUI compatibility for Window class:
 - Return zero for current size and position attributes when the window is
   closed.
 - Allow size and position attributes to be settable and use new values if
   the window is reopened.
 - Prevent windows whose position is given relative to a reference window
   from jumping down and to the right when reopened.
 - Open "centered" windows at the centre of the Intuition screen, not the
   centre of the overall display.
 - Refined behaviour of MUIV_Window_TopEdge_Delta to match MUI.
 - Windows opened at pointer coodinates now have their centre under the
   pointer, rather than having their top-left corner under the pointer.
 - Don't open an "under pointer" window at the top-left corner of its
   reference window if it has one.
 - Only save the position if the window has an ID.
 - Also save the position if the window was given special position values.
 - Added AutoDoc for MUIA_Window_DragBar.

git-svn-id: https://svn.aros.org/svn/aros/trunk/AROS@54752 fb15a70f-31f2-0310-bbcc-cdcc74a49acc