Don't put event box in toplevel window

Remove an unnecessary toplevel widget.

This event box was originally added by Marco "to make it easier
to take screenshot of the canvas area only" (7f731457c2) but we're
unsure why this is, and it doesn't seem to be needed for our current
screenshot-taking mechanism. Screenshots continue to work fine after
removing this.

src/sugar/graphics/window.py

@@ -103,12 +103,9 @@ class Window(gtk.Window):
         self.__vbox.pack_start(self.__hbox)
         self.__hbox.show()
 
-        self._event_box = gtk.EventBox()
-        self.__hbox.pack_start(self._event_box)
-        self._event_box.show()
-        self._event_box.add_events(gtk.gdk.POINTER_MOTION_HINT_MASK
-                                   | gtk.gdk.POINTER_MOTION_MASK)
-        self._event_box.connect('motion-notify-event', self.__motion_notify_cb)
+        self.add_events(gtk.gdk.POINTER_MOTION_HINT_MASK
+                        | gtk.gdk.POINTER_MOTION_MASK)
+        self.connect('motion-notify-event', self.__motion_notify_cb)
 
         self.add(self.__vbox)
         self.__vbox.show()
@@ -173,10 +170,10 @@ class Window(gtk.Window):
 
     def set_canvas(self, canvas):
         if self._canvas:
-            self._event_box.remove(self._canvas)
+            self.__hbox.remove(self._canvas)
 
         if canvas:
-            self._event_box.add(canvas)
+            self.__hbox.pack_start(canvas)
 
         self._canvas = canvas
         self.__vbox.set_focus_child(self._canvas)