QmlDesigner: Add empty placeholder to effect maker's view

Fixes: QDS-10592
Change-Id: Icc6193b358b83fce2a229321e30880e35e074055
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>

share/qtcreator/qmldesigner/effectMakerQmlSources/EffectMaker.qml

@@ -136,4 +136,17 @@ Item {
             } // Column
         } // ScrollView
     }
+
+    Text {
+        id: emptyText
+
+        text: qsTr("Add an effect node to start")
+        color: StudioTheme.Values.themeTextColor
+        font.pixelSize: StudioTheme.Values.baseFontSize
+
+        x: scrollView.x + (scrollView.width - emptyText.width) * .5
+        y: scrollView.y + scrollView.height * .5
+
+        visible: EffectMakerBackend.effectMakerModel.isEmpty
+    }
 }

share/qtcreator/qmldesigner/propertyEditorQmlSources/imports/HelperWidgets/Section.qml

@@ -177,7 +177,7 @@ Item {
         IconButton {
             id: closeButton
 
-            icon: StudioTheme.Constants.close_small
+            icon: StudioTheme.Constants.closeCross
             buttonSize: 22
             iconScale: containsMouse ? 1.2 : 1
             transparentBg: true

src/plugins/qmldesigner/components/effectmaker/effectmakermodel.cpp

@@ -56,12 +56,22 @@ bool EffectMakerModel::setData(const QModelIndex &index, const QVariant &value,
     return true;
 }
 
+void EffectMakerModel::setIsEmpty(bool val)
+{
+    if (m_isEmpty != val) {
+        m_isEmpty = val;
+        emit isEmptyChanged();
+    }
+}
+
 void EffectMakerModel::addNode(const QString &nodeQenPath)
 {
     beginInsertRows({}, m_nodes.size(), m_nodes.size());
     auto *node = new CompositionNode(nodeQenPath);
     m_nodes.append(node);
     endInsertRows();
+
+    setIsEmpty(false);
 }
 
 void EffectMakerModel::moveNode(int fromIdx, int toIdx)
@@ -82,6 +92,9 @@ void EffectMakerModel::removeNode(int idx)
     m_nodes.removeAt(idx);
     delete node;
     endRemoveRows();
+
+    if (m_nodes.isEmpty())
+        setIsEmpty(true);
 }
 
 const QList<Uniform *> EffectMakerModel::allUniforms()

src/plugins/qmldesigner/components/effectmaker/effectmakermodel.h

@@ -43,6 +43,7 @@ public:
     bool setData(const QModelIndex &index, const QVariant &value, int role) override;
 
     bool isEmpty() const { return m_isEmpty; }
+    void setIsEmpty(bool val);
 
     void addNode(const QString &nodeQenPath);