C++-Settings/License template: Add %FILENAME%, %CLASS% placeholders.

... for doxygen. Change logic for known empty keywords to be replaced
by an empty string.

Task-number: QTCREATORBUG-1854

src/plugins/cpptools/cppmodelmanagerinterface.h

@@ -170,7 +170,7 @@ public:
                               const QString &fileName,
                               int line, int column);
 
-    static QString licenseTemplate();
+    static QString licenseTemplate(const QString &file = QString(), const QString &className = QString());
 
 private:
     CppModelManagerInterface *m_modelmanager;