Style.
I mean what's an "a"?
Renamed TrackListWidgetItem to Track.
Renamed TrackListWidget to TrackWidget.
Comment.
Renamed MP4BoxVersionChecker to MP4BoxChecker.
Style.
Removed unused #include.
Const.
Getting the MP4Box when constructing.

Probably not even that great of an idea, because it happens on the
UI thread.

But in general, the whole MP4Box command thing is still awkward
and not great in general.

Later :>
Style.
Shorter variable names.

That are still quite long.
Returning an empty string on MP4Box failure instead of just logging it.
Comments.
Removed an unused #define.
Style.
Removed qDebug() calls.
Shorter variable names.
More sensible program flow.
Style.