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- int values are accepted where a long is expected.
- int or long values are accepted where a float is expected.

- 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.

Fix reference count problem when returning an already existing Python object from the native object user data.

This reverts commit aa05370d.