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/cppfilesettingspage.h

@@ -60,7 +60,7 @@ struct CppFileSettings
 
     // Convenience to return a license template completely formatted.
     // Currently made public in
-    static QString licenseTemplate();
+    static QString licenseTemplate(const QString &file = QString(), const QString &className = QString());
 
     bool equals(const CppFileSettings &rhs) const;
 };