<bmx-tabs>
One panel showing at a time, with a strip of tabs to choose it.
The panel's content is written where the panel is, and the tab is drawn from this element's
label. There is no second element to keep in step and no pair of ids to wire up.
A badge is a short count or status, and it belongs to the panel rather than to the tab.
Give a panel a value when the label is likely to change — a translated label would otherwise change
what the tabs report.
Not reachable while it is disabled.
Arrow keys move between tabs and choose as they go. Tab moves into the panel.
Vertical, in the pill appearance, and choosing only on Enter:
Every property, event and method: https://docs.binarymission.co.uk/webcomponents/bmx/bmx-tabs.html