CUSTOM REACT: support column-group properties in the projection

[AlexKirkouski]

Part of #1666 (CUSTOM custom views — phase 4).

Description

A property drawn across a column object group (a property repeated per column object) is not handled by the React projection: values and presentation are keyed by row only, so column-group cells do not project or optimistically update correctly. (Editing such cells from a React custom view is also affected.)

Add column-group support so a React custom view can render and edit a property that spans a column group — addressing each cell by both its row object and its column object.

FORM sales 'Sales by month'
    OBJECTS p = Product
    OBJECTS m = Month                 // the column object group
    PROPERTIES sold(p, m) COLUMNS (m)  // one value per (product, month)
    DESIGN sales { salesBox { custom = 'Matrix'; } }
;

// illustrative: a cell is addressed by row (product) + column (month)
function Matrix({ data, controller }) {
  return data.p.list.map(row =>
    <tr key={row.value}>{/* render row.sold per column object m */}</tr>);
}

Reason

Column-group (cross-tab / matrix) layouts cannot be rendered by a React custom view today. Supporting them — keyed by row and column object — lets React components cover the same forms the native grid does.