Fix displaying covers from cover.jpg files

Currently, images from cover.jpg files in the currently playing file are unconditionally overwritten with embedded covers, so cover images are never displayed for files that don't have embedded cover info.

Differential Revision: https://phabricator.kde.org/D18885
2 jobs for Applications/18.12 in 5 minutes and 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1390
freebsd
freebsd stable-kf5-qt5 qt5.12

00:03:08

passed #1389
linux stable-kf5-qt5 qt5.10

00:05:25

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