Find tooltip/dnd widget running through container children in inverse order

Find tooltip/dnd widget running through container children in inverse order

Usually, educated GtkContainers' forall() implementation returns children
in an order that's safe for the default draw() implementation in GtkContainer.
So for widgets with some stacking notions (eg. GtkOverlay),
_gtk_widget_find_at_coords() needs to recurse within containers in reverse
order so it finds the topmost widget.

As this function is used in both tooltips and DnD code, this improves behavior
of "floating" widgets wrt those two. This could for example be seen in the
"Transparent" GTK+ demo, where dropping text on the entry results on the text
going to the textview.

https://bugzilla.gnome.org/show_bug.cgi?id=699239