CppEditor: refactor FollowSymbol

Create an interface to get the ability to use
another FollowSymbol implementation

Change-Id: I5802f62523ff3ee47b8a14e487adf43edcb6c9b1
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/cppmodelmanagersupportinternal.cpp

@@ -69,3 +69,8 @@ CppCompletionAssistProvider *ModelManagerSupportInternal::completionAssistProvid
 {
     return m_completionAssistProvider.data();
 }
+
+FollowSymbolInterface *ModelManagerSupportInternal::followSymbolInterface()
+{
+    return nullptr;
+}