window leaks from gtk_drag_set_icon_widget()

[Tim Janik <timj gtk org>]

hi owen,

you might have noticed (assuming you have export GTK_DEBUG=objects in
your .profile ;) that most DnD operations leak winodws with ref counts
of 1.
this is due to code similar to

static void
gtk_color_selection_drag_begin (GtkWidget      *widget,
                                GdkDragContext *context,
                                gpointer        data)
{
  [...]
  window = gtk_window_new (GTK_WINDOW_POPUP);
  [...]
  gtk_drag_set_icon_widget (context, window, -2, -2);
  /* i.e. set and forget about the window */
}

for the colorselection, i just hooked up a drag_end handler that will
care about the window's destruction, so at least the colorselection
doesn't leak windows anymore.
a bunch of other code that does similar things in their drag_begin
handler probably needs to be adapted to care about the destruction
as well.
but given that the DnD internals do the same thing in
gtk_drag_set_icon_pixmap(), i'd rather see us taking a different path,
i.e.
introduce

/* Set the image window being dragged around, the window will
 * be auto-destructed at the end of the drag.
 */
void gtk_drag_set_icon_window  (GdkDragContext    *context,
                                GtkWidget         *widget,
                                gint               hot_x,
                                gint               hot_y);

which will auto-destroy the window in gtk_drag_remove_icon().
if people want to keep around the window for further drag operations,
they can still use gtk_drag_set_icon_widget(). for a related matter,
does it make sense at all to pass anything else than a window widget
into gtk_drag_set_icon_widget? i presume not, and so we should probably
add an appropriate g_return_if_fail statement.

---
ciaoTJ