Replace "expose-event" signal by a new "draw" signal

GtkWidget "expose-event" signal has been replaced by
a new "draw" signal [1]. The context is already
clipped [2], so do not base it on the values returned by
get_allocation like before.

[1] http://developer.gnome.org/gtk3/3.0/ch25s02.html#id1467092
[2] http://developer.gnome.org/gtk3/3.0/GtkWidget.html#GtkWidget-draw

Signed-off-by: Simon Schampijer <simon@schampijer.de>
[squashed with a patch by Benjamin Berg <benjamin@sipsolutions.net>;
 removed useless additions]
Signed-off-by: Sascha Silbe <silbe@activitycentral.com>

src/sugar3/graphics/icon.py

@@ -385,7 +385,7 @@ class Icon(Gtk.Image):
             width = 0
         return (width, width)
 
-    def do_expose_event(self, event):
+    def do_draw(self, cr):
         """
         Parameters
         ----------
@@ -414,8 +414,6 @@ class Icon(Gtk.Image):
         y = math.floor(ypad +
             (allocation.height - requisition.height) * yalign)
 
-        cr = self.get_window().cairo_create()
-
         if self._scale != 1.0:
             cr.scale(self._scale, self._scale)