title: Widgets in a Tkinter popup taking Tab focus from widgets in another window
body: <p>I am noticing an issue in my Tkinter application when I create a new Toplevel popup (actually a subclass of tkSimpleDialog.Dialog) and try to navigate through its widgets with the Tab key.  It works as expected, except whatever I had selected in a Listbox in my application's main window becomes unselected, as if the widget in the popup took the focus from it. Does anyone know why this is happening and how to prevent it? My Tkinter knowledge doesn't cover how interactions between windows affect the focus...</p>