v2.1.0

This release brings a few new & interesting features and improvements:

• Add SDBUSCPP_REGISTER_STRUCT macro to conveniently teach sdbus-c++ about user-defined structs
• Extend the SDBUSCPP_REGISTER_STRUCT macro with serialization of user-defined structs as dicts, and deserialization of dicts into user-defined structs
• Make createPlainMessage() function public
• Solve the problem of sending large D-Bus messages properly (through the event loop thread)
• Fix partially renamed BUILD_DOXYGEN_DOC CMake option
• Change googletest to default version 1.14.0
• Add version parameter to the xml2cpp codegen tool
• A few other internal refactorings and improvements