Drag & Drop: add overlapping feature.

Drag & Drop: add overlapping feature.

This feature is essential if two overlapping widgets can receive drop
information.
Until now, if two widgets (e.g background and button) were added as drop
targets, pointing to common coordinates would have given priority to the
first inserted as drop target.
Now, it will determine which widget is supposed to receive this drop
target by using the same mechanism as used for mouse move, i.e by
walking on the objects tree whose pointer passes through.

A test has been added (Overlapping DnD) to show how this feature can be
used. You can drop in bg, box and button.