"Extended tags" deletes embedded covers if multiple files with differnt albumart are selected

[dfrein]

Editing tags via "extended tags" deletes embedded album art if multiple files with different covers are selected - this destructive behaviour is not expected.

When selecting multiple files and then performing some tag changes with the "extended tag" tool, none of the embedded cover artworks are shown in this dialogue, but also no hint or warning that multiple covers are available. After performing tag changes in "extended tags" not related to album artwork, all of the previously embedded covers are removed without further warnings. This was tested with puddletag 2.0.1 (and github master) and different flac and mp3 files.