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.

Inizia dall’issue #77791 e dal commento collegato, quindi esamina la visualizzazione del contesto del codice, il menu e i punti di ingresso dell’editor di IDLE. Esegui o leggi test_codecontext e rivedi la guida delle impostazioni e i riferimenti alla documentazione. Il lavoro è completato quando ogni elemento elencato è stato verificato o deciso e il comportamento documentato è allineato con i test interessati.