def do_attach(self):
pass<EOL>
React to the widget being attached to the GUI.

Specifically, this method is called when the widget becomes connected, 
through any number of parent widgets, to the root of the widget 
hierarchy.  When this happens, ``self.root`` will return a widget 
rather than None.

Note that this method will not necessarily be called when the widget is 
simply added to a parent widget.  If the parent is already attached to 
the GUI itself, then this method will be called right then.  Otherwise, 
this method will be called when that parent (or one of its parents) is 
attached to the GUI.

This method is mainly useful for widgets that need to take control away 
from the root widget for some reason.  For example, Viewport widgets 
override how mouse events are interpreted and Dialog widgets grab all 
the key and mouse events for themselves.