Add XSPF in macos gui

Opening is implemented, but saving is still to be done. The user should be able to decide between m3u and XSPF in the Save-dialogue.

assigned to @fkuehne

Fixed in [15050]

added Status::fixed label

changed milestone to %0.8.5-test3

closed

changed the description

removed Status::fixed label

reopened

changed milestone to %0.8.5-final

added Type::feature label

A bit complex, according to bigben. Postponing.

changed milestone to %0.8.6

Implemented by bigben in [15445].

added Status::fixed label

closed