dlgtrackinfo - Updating member "sLoadedCoverLocation" after cover change
dlgtrackinfo - Add button to open the file browser in the tracklocation
if the cover comes from an external dir, we copy it to the track directory

This is important because the user could choose an image from a removable disk or any another place which could not be available in a “near future”.
So, it is much more safe let it together with the track.
It is also good because the user could reuse the cover in other software (automatic)…