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.

从 issue #77791 及其链接的评论开始，然后检查 IDLE 的代码上下文显示、菜单和编辑器入口点。运行或阅读 test_codecontext，并审查设置帮助和文档引用。每个列出的项目都已检查或作出决定，并且文档中描述的行为与受影响的测试一致，即表示完成。