Desktop: Accessibility: Improve note title focus handling

[personalizedrefrigerator]

Summary

This pull request implements suggestion 1 from the "How should Joplin handle keyboard navigation" forum post.

To summarize:

• The custom Tab keybinding override has been removed.
  • This keybinding override made it very difficult to focus the editor toolbar when navigating with a keyboard.
  • See WCAG §2.4.3 and (less relevant) WCAG §2.1.2.
• As a replacement, Enter and ArrowDown move the keyboard focus from the title to the editor.
  • ArrowDown moves the caret to the first line and focuses. Enter just focuses (& preserves the caret location).
  • In the legacy Markdown editor, ArrowDown just focuses (& preserves the caret location).
• ArrowUp on the first line of the editor moves to the title input.
  • This only applies to the new Markdown editor and the Rich Text Editor.

Related: #10795

Screen recording

Screencast.from.2024-08-26.15-32-57.webm

Testing plan

As shown in the screen recording above, this pull request has been tested manually on Fedora 40 (Linux):

1. Switch to the Rich Text Editor and focus the note title.
2. Press the Down arrow key.
3. Verify that the caret has moved to the first line of the editor.
4. Press the Up arrow key.
5. Verify that the caret has moved back to the note title.
6. Move the caret to the 2nd or 3rd line.
7. Focus the title.
8. Press Enter.
9. Verify that the caret has moved to the previously-focused paragraph.
10. Move the focus to the start of the editor.
11. Press Shift-Tab.
12. Verify that the focus has moved to the note toolbar.
13. Switch to the Markdown editor.
14. Focus the note title.
15. Press the Down arrow key.
16. Verify that the note editor's first line has focus.
17. Press Up.
18. Verify that the title has focus.
19. Press Enter.
20. Verify that the editor has focus.
21. Focus a line further from the beginning of the editor.
22. Focus the title input by clicking on it.
23. Press Down.
24. Type.
25. Verify that focus moved to the start of the editor.

Optional to-do: Add an automated Playwright test for the Markdown editor (currently, one only exists for the Rich Text editor).

[LeeBreisacher]

Is there any way to override/customize this change? I would really like to have the TAB key move from title to body.