The call-back function is also called when the window is created (if two are
used, one for moving and one for re-sizing, then both are called).
The call-back function will typically call the Clearwin+ function
get_window_location@ in order to determine the new size and position.
Note that certain window operations involve both moving and resizing. For
example if the user drags the top-left corner, then the window is both moved
and resized ('moving' changes the co-ordinates of the top left corner,
'resizing' changes the width and/or height).