AudioPlayer: display error when seek failure

As there is no 100% reliable way to determine is a file is seekable except by having it fail, the seek buttons cannot be hidden for unseekable files. Instead it will display an error when the seek fails. Closes #2137 (closed)

LGTM

added 12 commits

• 785650de...c258f1da - 11 commits from branch videolan:master
• b847a122 - AudioPlayer: display error when seek failure

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enabled an automatic merge when the pipeline for b847a122 succeeds

merged