a11y macOS: Support titleUIElement property

In the macOS accessibility bridge, bridge the QAccessible::Label
relation to the corresponding titleUIElement NSAccessibility property [1]:

> An element that represents another element’s static text title (id)

In case there are multiple targets for the QAccessible::Label relation,
just report the first one.

With this in place, Accessibility Inspector on macOS now e.g. shows
"Locale (text) [QMacAccessibilityElement]" as the "Title UIElement"
of the corresponding combobox it labels in the calendarwidgets example
as expected and VoiceOver now says e.g. "English/Germany Locale General
Options, menu button" instead of just the current combobox value and
role "English/Germany, menu button", so the context and semantics of
the combobox becomes clearer to the user.

[1] https://developer.apple.com/documentation/appkit/nsaccessibility-swift.struct/attribute/titleuielement

Change-Id: I814658628c481fe71f17853f71dc5690971507b1
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

src/plugins/platforms/cocoa/qcocoaaccessibilityelement.mm

@@ -539,6 +539,26 @@ static void convertLineOffset(QAccessibleTextInterface *text, int *line, int *of
     return nil;
 }
 
+- (id) accessibilityTitleUIElement {
+    QAccessibleInterface *iface = self.qtInterface;
+    if (!iface)
+        return nil;
+
+    const auto labelRelations = iface->relations(QAccessible::Label);
+    if (labelRelations.empty())
+        return nil;
+
+    QAccessibleInterface *label = labelRelations.first().first;
+    if (!label)
+        return nil;
+
+    QMacAccessibilityElement *accessibleElement = [QMacAccessibilityElement elementWithInterface:label];
+    if (!accessibleElement)
+        return nil;
+
+    return NSAccessibilityUnignoredAncestor(accessibleElement);
+}
+
 - (NSString*) accessibilityIdentifier {
     if (QAccessibleInterface *iface = self.qtInterface)
         return QAccessibleBridgeUtils::accessibleId(iface).toNSString();