Fix potential segmentation faults with exif. (#639)

Fix potential segmentation faults with exif. (#639)

1. A pointer exif_app_ to an ExifApp on QList<ExifApp> exif_apps was saved.
After this the list was modified.  This can lead to the saved pointer becoming
invalid.
2. The ExifApp structure had a dtor, but no other special functions, i.e.  it
violated the rule of 3 and the rule of 5.  Operations on QList<ExifApp> may
cause an ExifApp on the list to be copied or destroyed.  If an ExifApp is
destroyed, then the ExifApp dtor would close the files, even though there could
be a copy of the ExifApp expecting the files still to be open.

This scenerio occured with Qt6, causing segmentation faults in exif.test.