Share the move up and move down reordering logic of the class editor with Live Data

XWIKI-24755 adds move up and move down buttons to the class editor, so that XClass properties can be reordered without dragging. They are implemented by the xwiki-reorder-controls widget, a jQuery module served from the skin.

Live Data has the same need, for its column headers and for the column and sort panels, but it cannot use that widget: it is written in Vue, bundled from xwiki-platform-node, and it can neither load skin files nor rely on jQuery DOM moves. Without a shared implementation, Live Data would have to duplicate the logic, and the two editors would drift apart.

Proposal

• Extract the framework-agnostic parts into a small xwiki-platform-node package:
  • the move and boundary logic;
  • the arrow key mapping;
  • the live region announcing the new position to screen readers.
• Turn the current jQuery code into the class editor adapter of that package.
• Add a Vue adapter for Live Data later on.

Until then, the widget of XWIKI-24755 stays internal and is not advertised as a reusable API.

This was raised in the review of https://github.com/xwiki/xwiki-platform/pull/6267.