Support MainWindow being embedded into a widget on Linux/macOS too

Dragging indicators weren't being shown.

Suprisingly we already had a test for this: tst_embeddedMainWindow
But it passed on offscreen, just failed with xcb, which we don't
usually run

src/private/DragController.cpp

@@ -469,7 +469,7 @@ QWidgetOrQuick *DragController::qtTopLevelUnderCursor() const
         if (auto tl = qtTopLevelUnderCursor_impl(globalPos, DockRegistry::self()->nestedwindows(), m_windowBeingDragged->floatingWindow()))
             return tl;
 
-        return qtTopLevelUnderCursor_impl(globalPos, DockRegistry::self()->mainwindows(), static_cast<MainWindowBase*>(nullptr));
+        return qtTopLevelUnderCursor_impl(globalPos, DockRegistry::self()->topLevels(/*excludeFloating=*/true), static_cast<QWidget*>(nullptr));
     }
 #else
     // QtQuick: