pkg/lib: Give empty "Th" elements a aria-label attribute

As requested by this warning:

    Th: Table headers must have an accessible name. If the Th is
    intended to be visually empty, pass in screenReaderText. If the Th
    contains only non-text, interactive content such as a checkbox or
    expand toggle, pass in an aria-label.

pkg/lib/cockpit-components-table.jsx

@@ -259,9 +259,9 @@ export const ListingTable = ({
             <Table {...extraProps} {...tableProps}>
                 {showHeader && <Thead>
                     <Tr>
-                        {isExpandable && <Th />}
-                        {!onHeaderSelect && onSelect && <Th />}
-                        {onHeaderSelect && onSelect && <Th select={{
+                        {isExpandable && <Th aria-label={_("Expander")}/>}
+                        {!onHeaderSelect && onSelect && <Th aria-label={_("Select")}/>}
+                        {onHeaderSelect && onSelect && <Th aria-label={_("Select")} select={{
                             onSelect: onHeaderSelect,
                             isSelected: rows.every(r => r.selected)
                         }} />}