Added default to label arg in palette constructor

This leaves the API backward compatible but prevents the
need for new code to pass any unused arguments to the
constructor, instead depending on the property API.

sugar/graphics/palette.py

@@ -154,7 +154,7 @@ class Palette(gtk.Window):
 
     # DEPRECATED: label is passed with the primary-text property, accel_path 
     # is set via the invoker property, and menu_after_content is not used
-    def __init__(self, label, accel_path=None, menu_after_content=False,
+    def __init__(self, label=None, accel_path=None, menu_after_content=False,
                  text_maxlen=0, **kwargs):
 
         self.palette_state = self.PRIMARY