QmlDesigner: Disable apply material to selected when no selection exist

When there is no valid model selected, disable the "apply to selected"
actions in the material editor and browser.

Change-Id: Id8e771c64e69c0ba2f42dff01d19ffbf4afafb77
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/components/materialeditor/materialeditorcontextobject.cpp

@@ -233,6 +233,20 @@ void MaterialEditorContextObject::setHasQuick3DImport(bool b)
     emit hasQuick3DImportChanged();
 }
 
+bool MaterialEditorContextObject::hasModelSelection() const
+{
+    return m_hasModelSelection;
+}
+
+void MaterialEditorContextObject::setHasModelSelection(bool b)
+{
+    if (b == m_hasModelSelection)
+        return;
+
+    m_hasModelSelection = b;
+    emit hasModelSelectionChanged();
+}
+
 void MaterialEditorContextObject::setSelectedMaterial(const ModelNode &matNode)
 {
     m_selectedMaterial = matNode;