macOS: Activate Qt's platform a11y whenever the system queries us

Otherwise the cached information we store about e.g. table rows
and columns will not be updated, as QAccessible::isActive() is
false during QAbstractItemViewPrivate::rowsInserted(), and as a
result we'll be operating on stale information when the system
comes back later to query us about one of the cells.

This was observed when running a Qt Widgets table app with
the system's Keyboard Viewer open.

Change-Id: I88efd46dbc3d35c8b1888d3e29ef3d001bb9eac7
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>
(cherry picked from commit cf13e4c84c44b6f66f651035b3c3d012718b82dc)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/cocoa/qnsview_accessibility.mm

@@ -13,6 +13,15 @@
 
 @implementation QNSView (Accessibility)
 
+- (void)activateQtAccessibility
+{
+    // Activate the Qt accessibility machinery for all entry points
+    // below that may be triggered by system accessibility queries,
+    // as otherwise Qt is not aware that the system needs to know
+    // about all accessibility state changes in Qt.
+    QCocoaIntegration::instance()->accessibility()->setActive(true);
+}
+
 - (id)childAccessibleElement
 {
     QCocoaWindow *platformWindow = self.platformWindow;
@@ -32,8 +41,7 @@
 
 - (id)accessibilityAttributeValue:(NSString *)attribute
 {
-    // activate accessibility updates
+    [self activateQtAccessibility];
-    QCocoaIntegration::instance()->accessibility()->setActive(true);
 
     if ([attribute isEqualToString:NSAccessibilityChildrenAttribute])
         return NSAccessibilityUnignoredChildrenForOnlyChild([self childAccessibleElement]);
@@ -43,11 +51,13 @@
 
 - (id)accessibilityHitTest:(NSPoint)point
 {
+    [self activateQtAccessibility];
     return [[self childAccessibleElement] accessibilityHitTest:point];
 }
 
 - (id)accessibilityFocusedUIElement
 {
+    [self activateQtAccessibility];
     return [[self childAccessibleElement] accessibilityFocusedUIElement];
 }