Modify moz-fieldset to allow focusing the heading

For the bridge settings in the new design, we want a way to move the focus back to the start of a setting-group. The best option is the heading element within moz-fielset.

NOTE: Really, we want to be able to move the focus using a URL anchor, rather than tabindex="-1". However, about:preferences hijacks the anchor to do other things. See #43640 (closed).