qt: set application badge number to playlist count

Provides a nice way to show how many items left playing in the playlist.

Request review @chub.

added MRStatus::Reviewable label

added Component::Interface: Qt label

changed milestone to %4.0

badge is usually intended to show the presence/number of notifications. The user is usually expected to open the application do "things" to acknowledge the notifications to make the badge go away.

I feel this would be confusing or frustrating to some users

added MRStatus::NotCompliant label and removed MRStatus::Reviewable label

closed

mentioned in merge request !6340 (merged)