Reduce the number of torrent_handle::torrent_file() calls in QTorrentHandle::prioritize_files()

It was reported (#2161) that enabling/disabling a downloading of a file
is considerably slow on libtorrent 1.0.3, but not on 0.16.x. The problem
is that a function torrent_file() in libttorrent 1.0.3 does a deep copy
of torrent_info, while get_torrent_info() in libtorrent 0.16.x only
returns a reference.