Changed the way to find the drop widget from a top->bottom recursion
using GdkWindow positions to a liner bottom->top walk up the widget
hierarchy using _gtk_widget_find_at_coords() and
gtk_widget_translate_coordinates(), which both do the right things for
offscreen widgets.
(cherry picked from commit cb8c0763215bd18219c2d32b56e4bfe8a576f6b3)