Clear the setchpad after sending

sugar/chat/chat.py

@@ -70,6 +70,7 @@ class Chat(activity.Activity):
 		
 	def __send_button_clicked_cb(self, button):
 		self.send_sketch(self._sketchpad.to_svg())
+		self._sketchpad.clear()
 
 	def _color_selected(self, toolbox, color):
 		self._sketchpad.set_color(color)

sugar/chat/sketchpad/SketchPad.py

@@ -22,6 +22,10 @@ class SketchPad(gtk.DrawingArea):
 		self.connect("button-release-event", self.__button_release_cb)
 		self.connect("motion-notify-event", self.__motion_notify_cb)
 		self.connect('expose_event', self.expose)
+	
+	def clear(self):
+		self._sketches = []
+		self.window.invalidate_rect(None, False)
 
 	def expose(self, widget, event):
 		"""Draw the background of the sketchpad."""