a11y macOS: Report selection/Bridge QAccessibleSelectionInterface

On macOS, support NSAccessibilityProtocol's accessibilitySelectedChildren
method [1] by retrieving the selected children via the
QAccessibleSelectionInterface introduced in commit
9d16d5e224.

This e.g. makes a "selected children" attribute show up
in "UI Browser" [2] for the item views in the "interview" example
(examples/widgets/itemviews/interview/interview.app/Contents/MacOS/interview)
and that one has an array containing the selected items as value.

Sample scenario:

1) run the interview example (examples/widgets/itemviews/interview/interview.app)
2) select "Item 1:0", "Item 2:0" and "Item 3:0" by left-clicking
  on "Item 1:0" in the left view, then shift+clicking on "Item 3:0".
3) start UI Browser and navigate to the "group (group 1)" element
   and check that the value of its "selected children" attribute is an
   array containing the three selected items

[1] https://developer.apple.com/documentation/appkit/nsaccessibilityprotocol/1524672-accessibilityselectedchildren
[2] https://latenightsw.com/freeware/ui-browser/

Change-Id: Ic62f7fa82f2d01341a1dbc5ade2bd02c3ff99d9f
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/plugins/platforms/cocoa/qcocoaaccessibilityelement.mm

@@ -434,6 +434,30 @@ static void convertLineOffset(QAccessibleTextInterface *text, int *line, int *of
     return QCocoaAccessible::unignoredChildren(iface);
 }
 
+- (NSArray *) accessibilitySelectedChildren {
+    QAccessibleInterface *iface = QAccessible::accessibleInterface(axid);
+    if (!iface || !iface->isValid())
+        return nil;
+
+    QAccessibleSelectionInterface *selection = iface->selectionInterface();
+    if (!selection)
+        return nil;
+
+    const QList<QAccessibleInterface *> selectedList = selection->selectedItems();
+    const qsizetype numSelected = selectedList.size();
+    NSMutableArray<QMacAccessibilityElement *> *selectedChildren =
+            [NSMutableArray<QMacAccessibilityElement *> arrayWithCapacity:numSelected];
+    for (QAccessibleInterface *selectedChild : selectedList) {
+        if (selectedChild && selectedChild->isValid()) {
+            QAccessible::Id id = QAccessible::uniqueId(selectedChild);
+            QMacAccessibilityElement *element = [QMacAccessibilityElement elementWithId:id];
+            if (element)
+                [selectedChildren addObject:element];
+        }
+    }
+    return NSAccessibilityUnignoredChildren(selectedChildren);
+}
+
 - (id) accessibilityWindow {
     // We're in the same window as our parent.
     return [self.accessibilityParent accessibilityWindow];