libavformat/mxfdec.c: read project_name metadata

MXF files generated by Media Composer or LibMXF can contain a
project name property in the Preface. Lots of existing samples have them.

http://samples.ffmpeg.org/MXF/issue2160/PW0805A0V01.4C5B5636.EFA330.mxf
project_name    : DNX145 PW Test

http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket3450/WriteAvidMXFgenerated/5502_0010_v1.mxf
project_name    : Rombus

http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket3100/1sec_mxf_test_Video5270C795.mxf


project_name    : NVB_DOOD

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>