EffectMaker: Block adding same effect node twice

The actual blocking is done at uniform level, as the problem of having
same effect node twice is duplicate uniforms.

Fixes: QDS-11470
Change-Id: I77b15b4a207efaebff39b4f6b1700d70262abcdb
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>

src/plugins/effectmakernew/effectmakerwidget.cpp

@@ -76,6 +76,10 @@ EffectMakerWidget::EffectMakerWidget(EffectMakerView *view)
                         {"rootView", QVariant::fromValue(this)}});
     QmlDesigner::QmlDesignerPlugin::trackWidgetFocusTime(
         this, QmlDesigner::Constants::EVENT_NEWEFFECTMAKER_TIME);
+
+    connect(m_effectMakerModel.data(), &EffectMakerModel::nodesChanged, [this]() {
+        m_effectMakerNodesModel->updateCanBeAdded(m_effectMakerModel->uniformNames());
+    });
 }