QrCodeScannerPage: Always use background behind buttons

Using a background even if no explanation is shown improves the visibility of the button for skipping the scanning when the button is flat.
3 jobs for design/qr-code-scanner-page-with-button-background in 1 minute and 41 seconds
latest
Status Job ID Name Coverage
  Test
passed #62790
build

00:01:39

passed #62788
clazy

00:01:41

passed #62789
qmllint

00:00:30