audio/rosegarden, update to latest upstream release

Release notes at
https://rosegardenmusic.com/wiki/dev:20.06
various bugfixes.

On FreeBSD, with Qt 5.15, the "double-click-in-empty-space crash"
persists, mentioned in PR 242957. I haven't had time to debug
that (nor has the other participant in that thread).

Reported by: portscout