OOjs UI: FieldsetLayout: Styling fixes for `<legend>` labels

OOjs UI: FieldsetLayout: Styling fixes for `<legend>` labels

[Backport from OOjs UI master as this fixes a significant regression.]

* Make the `<legend>` 'display: inline-table' rather than 'table'.
  A block-level table would push down elements that follow it,
  including the "help" icon, which in turn messed up the fields.
  Fixes T145682, T145678 and first half of T145680.
* Only style .oo-ui-labelElement-label with margin and padding
  if we have the .oo-ui-labelElement class (label is not empty).
  Fixes second half of T145680.

Follow-up to 9cf8b941a9c618f1d7ba2512508f576f70116e1e.

Bug: T145682
Bug: T145678
Bug: T145680
Change-Id: Id95f4167afa6c000d6d2fa2ad7a62b65b6d4d53b