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-tfer.h

@@ -0,0 +1,159 @@
+/* 
+ * This file is part of Tox-WeeChat.
+ *
+ * Tox-WeeChat is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Tox-WeeChat is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Tox-WeeChat.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef TOX_WEECHAT_TFER_H
+#define TOX_WEECHAT_TFER_H
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+
+#include <tox/tox.h>
+#include <weechat/weechat-plugin.h>
+
+#include "twc-list.h"
+#include "twc-profile.h"
+
+#define TWC_TFER_LEGEND_LINES (4)
+#define TWC_TFER_FILE_STATUS_MAX_LENGTH (256)
+#define TWC_MAX_CHUNK_LENGTH (1371)
+#define TWC_MAX_SIZE_SUFFIX (5)
+#define TWC_MAX_SPEED_SUFFIX (5)
+
+enum t_twc_tfer_file_status
+{
+    TWC_TFER_FILE_STATUS_REQUEST,
+    TWC_TFER_FILE_STATUS_IN_PROGRESS,
+    TWC_TFER_FILE_STATUS_PAUSED,
+    TWC_TFER_FILE_STATUS_DONE,
+    TWC_TFER_FILE_STATUS_DECLINED,
+    TWC_TFER_FILE_STATUS_ABORTED,
+};
+
+enum t_twc_tfer_file_type
+{
+    TWC_TFER_FILE_TYPE_DOWNLOADING,
+    TWC_TFER_FILE_TYPE_UPLOADING,    
+};
+
+struct t_twc_tfer_file
+{
+    enum t_twc_tfer_file_status status;
+    enum t_twc_tfer_file_type type;
+    uint64_t position;  /* already transmitted (in bytes) */
+    uint64_t size;
+    uint32_t friend_number;
+    uint32_t file_number;
+    FILE *fp;
+    char *filename;
+    char *full_path;
+    char *nickname;
+    double timestamp;
+    float cached_speed;
+    size_t after_last_cache;
+};
+
+struct t_twc_tfer
+{
+    struct t_twc_list *files;
+    struct t_gui_buffer *buffer;
+    char *downloading_path;
+};
+
+int
+twc_tfer_buffer_input_callback(const void *pointer, void *data,
+                               struct t_gui_buffer *weechat_buffer,
+                               const char *input_data);
+int
+twc_tfer_buffer_close_callback(const void *pointer, void *data,
+                               struct t_gui_buffer *weechat_buffer);
+struct t_twc_tfer *
+twc_tfer_new();
+
+enum t_twc_rc
+twc_tfer_load(struct t_twc_profile *profile);
+
+bool
+twc_tfer_has_buffer(struct t_twc_profile *profile);
+
+int
+twc_tfer_print_legend(struct t_twc_tfer *tfer);
+
+double
+twc_tfer_get_time();
+
+void
+twc_tfer_update_downloading_path(struct t_twc_profile *profile);
+
+char *
+twc_tfer_file_name_strip(const char *original, size_t size);
+
+struct t_twc_tfer_file *
+twc_tfer_file_new(struct t_twc_profile *profile,
+                  const char *nickname, const char *filename,
+                  uint32_t friend_number, uint32_t file_number,
+                  uint64_t size, enum t_twc_tfer_file_type filetype);
+
+void
+twc_tfer_file_add(struct t_twc_tfer *tfer, struct t_twc_tfer_file *file);
+
+uint8_t *
+twc_tfer_file_get_chunk(struct t_twc_tfer_file *file, uint64_t position, size_t length);
+
+bool
+twc_tfer_file_write_chunk(struct t_twc_tfer_file *file, const  uint8_t *data, uint64_t position, size_t length);
+
+struct t_twc_tfer_file *
+twc_tfer_file_get_by_number(struct t_twc_tfer *tfer, uint32_t file_number);
+
+size_t
+twc_tfer_file_get_index(struct t_twc_tfer *tfer, struct t_twc_tfer_file *file);
+
+void
+twc_tfer_file_update(struct t_twc_tfer *tfer, struct t_twc_tfer_file *file);
+
+int
+twc_tfer_file_accept(struct t_twc_profile *profile, size_t index);
+
+int
+twc_tfer_file_decline(struct t_twc_profile *profile, size_t index);
+
+int
+twc_tfer_file_pause(struct t_twc_profile *profile, size_t index);
+
+int
+twc_tfer_file_continue(struct t_twc_profile *profile, size_t index);
+
+int
+twc_tfer_file_abort(struct t_twc_profile *profile, size_t index);
+
+int
+twc_tfer_update_status(struct t_twc_tfer *tfer, const char *status);
+
+void
+twc_tfer_buffer_update(struct t_twc_tfer *tfer);
+
+void
+twc_tfer_file_err_send_message(char *message, enum TOX_ERR_FILE_SEND error);
+
+void
+twc_tfer_file_free(struct t_twc_tfer_file *file);
+
+void
+twc_tfer_free(struct t_twc_tfer *tfer);
+
+#endif /* TOX_WEECHAT_TFER_H */