Remove PassiveNotification in favor of Kirigami.InlineMessage

Summary:
Remove entirely the PassiveNotification and use the already
present Kirigami.InlineMessage to consistently display
stuff related to playlists

Test Plan:
load e.g. any file that is not a playlist
using the load playlist button, new notification is displayed

Reviewers: mgallien, ngraham

Reviewed By: ngraham

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D21911