IDLE: improve handling of int entry in settings dialog

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2020-01-27.23:34:59.719>
labels = ['expert-IDLE', 'type-bug', '3.9']
title = 'IDLE: improve handling of int entry in settings dialog'
updated_at = <Date 2020-01-28.12:32:21.133>
user = 'https://github.com/terryjreedy'

bugs.python.org fields:

activity = <Date 2020-01-28.12:32:21.133>
actor = 'cheryl.sabella'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2020-01-27.23:34:59.719>
creator = 'terry.reedy'
dependencies = []
files = []
hgrepos = []
issue_num = 39472
keywords = []
message_count = 2.0
messages = ['360821', '360867']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'cheryl.sabella']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39472'
versions = ['Python 3.9']