Font Axes click to reveal axis metadata: Show OT tag

When I'm working full screen with the Axes window in a 2nd window 2-up, I can see my axes metadata nicely:

I see there's a handy `info-box` class shown when you click the Axis Label in the Font Axes slider bank, and when I'm on a smaller screen with the Axes window as a tab say, I'd like to see the `OT tag` here too, like this:

I think the "UI name" (assuming that's what is in the fvar table, so what is shown in users' UIs) is fine to not show here, since we have the "Name" for our own type development work, which I guess can vary throughout a project, while the UI Name is going to be much more fixed.

I say this because sometimes project docs will refer to the axis OT tag, and that is more likely to be consistent, than the labels :)

Start from the Font Axes slider bank and the existing info-box behavior shown when clicking an Axis Label. Add the axis OT tag to that metadata display, then verify that clicking an Axis Label reveals it as shown in the example.

Written by the indexing model from the issue text.