IDLE: disable format menu items when not applicable

[roseman]

BPO	24817
Nosy	@terryjreedy, @roseman

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = <Date 2020-01-07.22:12:03.705>
created_at = <Date 2015-08-07.04:36:56.789>
labels = ['expert-IDLE', 'type-bug', '3.9']
title = 'IDLE: disable format menu items when not applicable'
updated_at = <Date 2020-01-07.22:12:03.688>
user = 'https://github.com/roseman'

bugs.python.org fields:

activity = <Date 2020-01-07.22:12:03.688>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = True
closed_date = <Date 2020-01-07.22:12:03.705>
closer = 'terry.reedy'
components = ['IDLE']
creation = <Date 2015-08-07.04:36:56.789>
creator = 'markroseman'
dependencies = []
files = []
hgrepos = []
issue_num = 24817
keywords = []
message_count = 2.0
messages = ['248171', '359553']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'markroseman']
pr_nums = []
priority = 'normal'
resolution = 'works for me'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue24817'
versions = ['Python 3.9']

[roseman]

In an editor window, with no selection, most of the items in the format menu (indent, tabify, etc.) aren't applicable, so the corresponding menu items should be disabled.

[terryjreedy]

The default selection is, now at least, the current line, so that every entry is always applicable.