This will find the window that the X and Y coordinates point to. This is very
useful when using Drag Gadgets to move icons from one window to another.
After the gadget has been dragged, use this call to find what window, if any,
the icon was dragged to. The window that the gadget belongs to is passed,
along with the coordinates of the gadget (in the window's coordinate system -
not the screen).
INPUTS
Window
Pointer to window that icon was dragged from.
X, Y
Position of icon relative to this window.
RESULT
Window
Pointer to the topmost window under the point, or NULL if no such
window exists.