- Add the LinphoneBuffer object so that file transfer callbacks do not pass out parameters
- Use the LinphoneBuffer object in the file transfer callbacks
- Implement the file_transfer_send callback in the Python file transfer test

Callbacks getters still need to be implemented correctly.

For functions returning or taking an MSList as an argument, specify what the MSList contains in the documentation.

Add attributes for classes in the XML file generated by the genapixml tool to tell whether a class is refcountable and/or destroyable.