'Initialize root, engine, and top attributes.
top (level widget): set in create_widgets() called from open().
text (Text being searched): set in open(), only used in subclasses().
ent (ry): created in make_entry() called from create_entry().
row (of grid): 0 in create_widgets(), +1 in make_entry/frame().
title (of dialog): class attribute, override in subclasses.
icon (of dialog): ditto, use unclear if cannot minimize dialog.'

def __init__(self, root, engine):

    self.root = root
    self.engine = engine
    self.top = None
