IDLE: More code_context improvements

Continuation of #77791. Below is https://github.com/python/cpython/issues/77791#issuecomment-1093786893 with done items removed. Items need to be checked or decided on if still relevant. New items should continue with next new number on original.

Docs:

• D3

Menu

• M4: Shortcut for CC? (new)

Code Context Display

• C1. Add vertical scrollbar if lines > maxlines (12).
• C2. Add horizontal scrollbar if add to text (13).
• C3: Make 'Show' show the context for the current top visible line (new).
• C5

Internal Changes

• I3

External Changes

• E2. Navigate by blocks in editor (14).
• E4. Max context lines = 0 is treated as infinity. For depth 0, "self.info[-self.context_depth:]" is whole slice. I don't remember if this is intended (if one does not want context, don't toggle it on), but it is not documented in settings help or doc section. In any case, offset = max(1, lines - self.context_depth) - 1 fails with depth 0.
• E5. test_codecontext is affected by user customization, but should not be.

まず issue #77791 とリンクされたコメントを確認し、続いて IDLE のコードコンテキスト表示、メニュー、エディターのエントリーポイントを調査します。test_codecontext を実行するか読み、設定ヘルプとドキュメント参照を確認します。各項目について確認または判断が行われ、文書化された動作と影響を受けるテストが一致していれば完了です。