IDLE, configdialog: Factor out ExtPage class from ConfigDialog

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2017-08-15.01:52:03.762>
labels = ['expert-IDLE', '3.7']
title = 'IDLE, configdialog: Factor out ExtPage class from ConfigDialog'
updated_at = <Date 2017-08-15.01:53:21.423>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2017-08-15.01:53:21.423>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2017-08-15.01:52:03.762>
creator = 'terry.reedy'
dependencies = ['31093']
files = []
hgrepos = []
issue_num = 31207
keywords = []
message_count = None
messages = []
nosy_count = 1.0
nosy_names = ['terry.reedy']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue31207'
versions = ['Python 3.6', 'Python 3.7']