QmlDesigner: Add setting for Qt 6 projects

Adding
'qt6Project: true'
to the qmlproject file marks the project as Qt 6 'only'.

Task-number: QDS-4734
Change-Id: Ib9d969fd11d409e280734f9bedf759b40470fc61
Reviewed-by: Henning Gründl <henning.gruendl@qt.io>

src/plugins/qmlprojectmanager/qmlproject.cpp

@@ -272,6 +272,13 @@ bool QmlBuildSystem::qtForMCUs() const
     return false;
 }
 
+bool QmlBuildSystem::qt6Project() const
+{
+    if (m_projectItem)
+        return m_projectItem.data()->qt6Project();
+    return false;
+}
+
 void QmlBuildSystem::setMainFile(const QString &mainFilePath)
 {
     if (m_projectItem)
@@ -466,6 +473,8 @@ QVariant QmlBuildSystem::additionalData(Id id) const
         return forceFreeType();
     if (id == Constants::customQtForMCUs)
         return qtForMCUs();
+    if (id == Constants::customQt6Project)
+        return qt6Project();
     return {};
 }