tox: Don't forget to destroy file transfers

We also need to destroy all file transfers when disposing a NeulandTox
instance.
file transfer: Fix some memory leaks
file transfer: Remove unneeded line

We already set the state at the end of the function in case an error
occurred.
file transfer: Harmonize functions

Harmonize and improve code and error handling in _append_data() and
_get_next_data().
file transfer: Change type of priv member

Make priv->input_stream and priv->output_stream types consistent.
file transfer: transferred-size property changes

Remove the _set_ function and replace it by an _add_ function, and put
the calculations whether to notify about the property change there. We
currently notify on every 1000th part of the file that has been
transmitted. Now we consistently print debug messages for the progress
of incoming and outgoing file transfers.