[#2406] [Core] [GTKUI] Implement core.add_torrent_files

 * Speeds up adding multiple torrents
[#2250] [Core] [GTKUI] Added method remove_torrents to core

Removing multiple torrents with remove_torrents will be faster
than calling remove_torrent for each torrent, as the state file
will be written only once, after all the torrents are deleted.
[#2250] [WebUI] [Console] Use new core.remove_torrents method