Adapt to Widget.get_child_requisition() API changes

In PyGTK Widget.get_child_requisition() returned a tuple [1]. In GTK3+pygi
a Requisition object is returned instead.

Based on a patch by Benjamin Berg <benjamin@sipsolutions.net>.

[1] http://developer.gnome.org/pygtk/stable/class-gtkwidget.html#method-gtkwidget--get-child-requisition
[2] http://developer.gnome.org/gtk/stable/GtkWidget.html#gtk-widget-get-child-requisition

Signed-off-by: Sascha Silbe <silbe@activitycentral.com>

src/sugar3/graphics/icon.py

@@ -409,9 +409,9 @@ class Icon(Gtk.Image):
 
         allocation = self.get_allocation()
         x = math.floor(allocation.x + xpad +
-            (allocation.width - requisition[0]) * xalign)
+            (allocation.width - requisition.width) * xalign)
         y = math.floor(allocation.y + ypad +
-            (allocation.height - requisition[1]) * yalign)
+            (allocation.height - requisition.height) * yalign)
 
         cr = self.get_window().cairo_create()