Add cover art from file metadata is not saved (tested for MP4 and MKV)

The behavior of the metadata editor is confusing. It would be better to have the Save Metadata always be present and greyed out until a change is made rather than hidden until a change is made.

Even more confusing is that the Save Metadata button does not appear after an image is added by the 'Add cover art from file' context menu item. Moreover if another item, e.g. 'Artist' is altered to force the Save Metadata button to appear, the album art is not saved (the text based 'Artist' does get saved). I've tested this for MP4 (H264 - MPEG-4 AVC + ACC) and MKV files (H.265 + ACC).