fix import torrent with "Keep incomplete torrents in:" ticked

* also had to account for "Append the label of the torrent to the save path",
  but again, this was only an issue when "Keep incomplete torrents in:" is
  selected

* A multi-file torrent with only one file (ie: a single file within a folder),
  was being treated as a single-file torrent, making it impossible to import.
  Multi-file torrent detection code was copied from libtorrent.  The
  information is available in libtorrent (under torrent_info::m_multifile),
  however it's a private member and I chose to go with copying the code that
  determines it, rather than modifying a library qBittorrent depends on.
Don't listen on IPv6 address by default. Prevents network connectivity problems. Closes #1880.

Conflicts:
	src/preferences/advancedsettings.h
	src/preferences/preferences.cpp
	src/preferences/preferences.h
	src/qtlibtorrent/qbtsession.cpp
Split ShutdownConfirmDlg into .h/.cpp files.
Added 'Shutdown now' button in shutdown confirmation dialog. Closes #969.

Conflicts:
	src/qtlibtorrent/qbtsession.cpp
Sync translations from Transifex and run lupdate.
Fix libtorrent 0.15.x compilation.
Set 'Cancel' as the default button in the shutdown confirmation dialog.
Don't disable DHT/LSD/UPnP in the GUI when anonymous mode is enabled.

Conflicts:
	src/preferences/options_imp.cpp
Removed broken Report a bug iframe from the webui. fixes qbittorrent/qBittorrent#1343
Removed broken "Documentation" iframe from the webui. Improves fix qbittorrent/qBittorrent#1343
Removed essentially useless "Visit website" iframe and changed it to a regular link. Improves fix qbittorrent/qBittorrent#1343
Possible fix for crash in RSS code. Closes #1901.
add host address parameter to helo smtp request

Conflicts:
	src/smtp.cpp
Right align numerical values in the transfer list. Partially revert 51a9a9aab. Closes #1903.
Add peer port column to PeerListWidget. Closes #1650.
Don't display the context menu in 'Content' and 'HTTP Sources' buttons when no torrent is selected. It prevents crashes. Closes #1906, #1900.
Don't translate file extensions. Closes #1907.
Fix available disk space label on single file torrents.
SMTP: Support multiline greeting from server in INIT state. Thanks to Ivanov Juriy for helping identify the problem. Relates to #1845.
SMTP: Send the IP of the configured Network Interface. Relates to #1845.

Conflicts:
	src/smtp.cpp