QmlDesigner: Compile with Qt 6

Qt 6 does cannot handle forward-declared types in signal declarations.

Change-Id: Iad4a810841df3d077750ad76c892bdb7eecc0a80
Reviewed-by: Knud Dollereder <knud.dollereder@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmldesigner/components/curveeditor/detail/selectionmodel.h

@@ -25,13 +25,13 @@
 
 #pragma once
 
+#include "curveitem.h"
 #include "treeitem.h"
 
 #include <QItemSelectionModel>
 
 namespace DesignTools {
 
-class CurveItem;
 class TreeItem;
 class NodeTreeItem;
 class PropertyTreeItem;

src/plugins/qmldesigner/components/curveeditor/detail/treeview.h

@@ -25,6 +25,7 @@
 
 #pragma once
 
+#include "curveitem.h"
 #include "selectionmodel.h"
 
 #include <QTreeView>
@@ -33,7 +34,6 @@ namespace DesignTools {
 
 class AnimationCurve;
 class CurveEditorModel;
-class CurveItem;
 
 struct CurveEditorStyle;