widget.Table: show first row/column when navigating with keyboard

[pierrec]

Fixes #5088.

Description:

When a Table has its header(s) shown, keyboard navigation fails to show the first row or column if the list was scrolled. This is because the header size is always added to the offset even when the first row/column is reached. Disabling this for the first row or column fixes the issue.

Fixes #5088

Checklist:

• [X] Tests included.
• [X] Lint and formatter run with no errors.
• [X] Tests all pass.

[pierrec]

I tested by moving too fast, I see the issue now. Will get back to it.

[andydotxyz]

@pierrec can you look at these failures? Because of the force-push it has lost the build history so it is not possible to see where it broke. Please do avoid force-push after reviews have started so we get more info on what is changing.

[pierrec]

@andydotxyz closing this PR and resubmitting on lastest develop branch.