tox: Improve neuland_tox_set_data_path()

Try to be better at error handling and in deciding when to set the
default name and status message.
Clean up debug messages a bit

Don't use g_message where g_debug is sufficient, and use g_warning when
appropriate.
Allow message lengths > TOX_MAX_MESSAGE_LENGTH

To make this possible we send the message in pieces, properly splitting
it at UTF-8 characters boundaries. Before this change sending the
message failed silently without informing the user.
contact: Count chars not bytes for preferred name
tox: Improve debug message

Use the first 10 chars (not bytes, as before) of a message or action in
the debug message for sending messages or actions.