TreeModel: Introduce a StaticTreeItem

This splits out the case of static string displays from the
TreeItem base class, making the base more lightweight.

Change-Id: If1f442011ec60094399a41b65d9b5015f432f82e
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/autotest/testtreeitem.cpp

@@ -38,8 +38,7 @@ namespace Autotest {
 namespace Internal {
 
 TestTreeItem::TestTreeItem(const QString &name, const QString &filePath, Type type)
-    : TreeItem( { name } ),
-      m_name(name),
+    : m_name(name),
       m_filePath(filePath),
       m_type(type),
       m_line(0),