Shortcuts for navigating forward and backward between recently focused documents

## Problem

It is quite common when developing to have lots of tabs open but mainly jump backwards between two or three documents. It would be useful to be able to do this quickly with keyboard shortcuts (and/or navigation buttons à la Files) rather than hunting for the desired tab or sidebar entry.

## Proposal

Introduce a document focus history and add actions to go backwards and forwards in it linked to keyboard shortcuts e.g. `LeftArrow`and `RightArrow`

## Prior Art

Geany has buttons to jump forward and backward between locations, although they do not always work as expected. This proposal would *only* change the focused document.

The issue names no files, tests, or entry points. Start by locating the Code editor's document-focus history and keyboard-shortcut handling, then review how navigation actions are exposed. Done means users can move backward and forward through recently focused documents with the proposed shortcuts without changing document content.

Written by the indexing model from the issue text.