Implement texture context menu

Apply to selected model/material, delete, duplicate, and
create new options are available in the menu.

Fixes: QDS-8342
Change-Id: Ib9bdc1738500a87361000bcd3e89403e3b8ccef8
Reviewed-by: Mahmoud Badri <mahmoud.badri@qt.io>

src/plugins/qmldesigner/components/materialbrowser/materialbrowsermodel.cpp

@@ -315,6 +315,13 @@ ModelNode MaterialBrowserModel::materialAt(int idx) const
     return {};
 }
 
+ModelNode MaterialBrowserModel::selectedMaterial() const
+{
+    if (isValidIndex(m_selectedIndex))
+        return m_materialList[m_selectedIndex];
+    return {};
+}
+
 void MaterialBrowserModel::resetModel()
 {
     beginResetModel();