Support utf16 in qt metadata

[Carl Eugen Hoyos]

(videolan ticket 17608)
Attached mov file is supposed to show BjörŃ i SKõGẽÑ as copyright string.

$ ffmpeg -i sample.mov
ffmpeg version N-82963-g9ec52a0 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      55. 43.100 / 55. 43.100
  libavcodec     57. 70.100 / 57. 70.100
  libavformat    57. 61.100 / 57. 61.100
  libavdevice    57.  2.100 / 57.  2.100
  libavfilter     6. 68.100 /  6. 68.100
  libswscale      4.  3.101 /  4.  3.101
  libswresample   2.  4.100 /  2.  4.100
  libpostproc    54.  2.100 / 54.  2.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'sample.mov':
  Metadata:
    major_brand     : qt
    minor_version   : 512
    compatible_brands: qt
    title           : SÚck THÌS
    title-fra       : SÚck THÌS
    artist          : RèFp
    artist-eng      : RèFp
    copyright       : ��
    copyright-swe   : ��
  Duration: 00:00:00.32, start: 0.000000, bitrate: 204 kb/s
    Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 43 kb/s, 50 fps, 50 tbr, 12800 tbn, 100 tbc (default)
    Metadata:
      handler_name    : DataHandler
      encoder         : Lavc57.64.100 libx264
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 116 kb/s (default)
    Metadata:
      handler_name    : DataHandler
At least one output file must be specified