Add RedoButton accelerator

- use ctrl+y to be consistent with Write Activity,

- a design discussion gave no responses,
  http://lists.sugarlabs.org/archive/sugar-devel/2020-March/057832.html

Signed-off-by: James Cameron <quozl@laptop.org>

src/sugar3/activity/widgets.py

@@ -108,6 +108,7 @@ class RedoButton(ToolButton):
     def __init__(self, **kwargs):
         ToolButton.__init__(self, 'edit-redo', **kwargs)
         self.props.tooltip = _('Redo')
+        self.props.accelerator = '<Ctrl>Y'
 
 
 class CopyButton(ToolButton):