bug#58724: Consider showing the vertical border in the tab line

[Juri Linkov]

> The vertical border is not shown in the tab line. See
> no-window-divider-mode.png for an example. On the other hand, the
> window divider is (when using window-divider-mode). See
> window-divider-mode.png for an example.
>
> I think it would make sense to show the vertical border in the tab
> line, since it is still part of the window. On the other hand, the
> mode line is also part of the window but the vertical border is not
> shown there.

This is the reason why the vertical border is not shown in the tab line -
for consistency with the mode line and the header line.

> While it is possible to customize tab line to do this,
> this is a minor UI detail that could be there from the start. Other
> editors show their corresponding window dividers in their
> corresponding tab line, which leads to a more consistent UI (albeit a
> small detail).

Patches welcome to show the vertical border everywhere:
in the mode line, header line, tab line.