Add AA_DontUsePopupWindows

We want QQuickPopup to open a top-level window in qt quick, by default.
But also allow users to opt out, and revert back to the old behavior.

It makes sense to have an application wide setting for this, in order
to allow application developer to opt out of the new default behavior,
without having to change a property for every instance of Popup.

While QCoreApplicationPrivate::attribs bearly has any available bits,
it appear that bit 30 is available, since
AA_DisableWindowContextHelpButton was commented out.

Task-number: QTBUG-121363
Change-Id: I6b2759d89b46aa5d08222c2611eaffe855cc425c
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/corelib/global/qnamespace.h

@@ -461,7 +461,7 @@ namespace Qt {
         AA_DisableShaderDiskCache = 27,
         AA_DontShowShortcutsInContextMenus = 28,
         AA_CompressTabletEvents = 29,
-        // AA_DisableWindowContextHelpButton = 30,
+        AA_DontUsePopupWindows = 30,
         AA_DisableSessionManager = 31,
 
         // Add new attributes before this line

src/corelib/global/qnamespace.qdoc

@@ -279,6 +279,11 @@
            level windows, unless required by the implementation.
            This value was added in Qt 6.8.
 
+    \value AA_DontUsePopupWindows When this attribute is set, popups will always appear
+           as items in the scene, rather than having their own dedicated windows.
+           Setting this attribute will only affect Qt Quick applications.
+           This value was added in Qt 6.8.
+
     \omitvalue AA_AttributeCount
     \omitvalue AA_EnableHighDpiScaling
     \omitvalue AA_UseHighDpiPixmaps