NPAL19051:
1) Parameter 'wid' is added in windowList() method. It specifies area which windows have to be returned; if it is equal to null when widgets of all areas are retuned
2)New method move() is added for moving the first widget to the same area which the second widget belongs to.
3)New method stack() is added for grouping all windows in one area.
4)New auxiliary private method wgArea is added for detecting area containing given widget
