[BUG] arrow keys no longer advance through file list, as of 4.5.7

[hqm]

Describe the bug In the File List view, the up and down arrow keys no longer advance to next/prev file

To Reproduce Steps to reproduce the behavior:

1. Go to 'File List' view
2. Click on a file
3. Use down arrow key to advance to visit next file
4. System does not advance to next file

Expected behavior In 4.5.6 the up and down arrow key used in the File List would load prev/next file into labelme

Desktop (please complete the following information):

• OS: Big Sur 11.2.3
• Labelme Version 4.5.7
• Python 3.8

Additional context I loaded version 4.5.7 using pip install, updating from 4.5.6 and noticed this issue

[hqm]

I found a workaround, the "D" and "A" keys behave like the arrow keys used to

[wkentaro]

Actually, "A" and "D" are officially supported ways of navigating in the file list. I'm not sure if we should keep supporting arrow keys as well since it could be also using moving polygons or the scroll bar.