query.Query creates a popup with entry and ok/cancel buttons, in that order.  Tabbing moves from the entry in that order.  Currently, subclasses that add widgets add them after the 3 above, so they follow Cancel in the tab order.  They do this by overriding create_widgets and initially calling super.create_widgets.

Added widgets should follow the entry box and precede the exit buttons.  To do this, they should be created in between.  Proposed solution: create_widgets calls 'extra_widgets' (pass for Query) after creating the Entry.  Subclasses override to add widgets.