Add support for mp4 "shwm" tag (proprietary iTunes flag)

This tag only exists in mp4 files and has no pendant in mp3 files. When set, iTunes shows "movement and work" instead of "song and group". When this mode is switched off, iTunes removes the "shwm" tag instead of setting it to zero.

(For mp3 files iTunes remembers internally how to show them, instead of writing it to the file, further it mixes GRP1 and TIT1 and causes a total mess)