Implement file transfer feature

This will add separate "tfer" buffer with all necessary controls for
managing files and /send command to start transfer, which you can use
either from profile buffer or from private messages buffer; in case of
incoming files the buffer will be created as well if doesn't exist yet.
Every callbacks were defined according to the specification and this
implementation is fully corresponding to the documented behavior including
streams.

src/twc-utils.h

@@ -58,4 +58,19 @@ twc_fit_utf8(const char *str, int max);
 int
 twc_set_buffer_logging(struct t_gui_buffer *buffer, bool logging);
 
+char *
+twc_tox_err_file_control(enum TOX_ERR_FILE_CONTROL error);
+
+char *
+twc_tox_err_file_get(enum TOX_ERR_FILE_GET error);
+
+char *
+twc_tox_err_file_seek(enum TOX_ERR_FILE_SEEK error);
+
+char *
+twc_tox_err_file_send(enum TOX_ERR_FILE_SEND error);
+
+char *
+twc_tox_err_file_send_chunk(enum TOX_ERR_FILE_SEND_CHUNK error);
+
 #endif /* TOX_WEECHAT_UTILS_H */