Fix stem destruction

Currently, when adding a lot of metadata so that the space in free atoms does not suffice, jaudiotagger effectively destroys Native Instrument STEM files, because it only adjust the first stco atom, and not the stco atoms of the other 4 tracks contained in the file. This leads to NI Traktor not being able to play those STEM files anymore, because it cannot find the STEMS/streams/tracks (the direct offsets in the other 4 stco atoms are wrong/not adjusted). This leaves STEM owners kinda angry...

This path fixes the problem.

• Improved comments and formatting (mostly cosmetic).

• Improved comments and formatting (purely cosmetic).

• Correctly adjust all stco atoms (not just the first) that stem from multiple trak atoms as they occur in Native Instruments STEM files. Without this fix, only the first stco atom (direct offset into the media stream), is adjusted and the other tracks/stream cannot be found.

• Additional test for stem.mp4.