Summary:
now adding albums from AllAlbumsProxyModel is fast

when adding tracks from the proxy models, use a working thread and
signals

this means that the main thread and the interface is no longer blocked

adds new methods to MediaPlayList to add list of albums or list of
tracks or list of artists

adding a lot of tracks or artists is still very slow and blocks the
interface

include QtConcurrentRun instead of QtConcurrent (suggested by ebn.kde.org)

enqueue a lot of tracks is now much faster

Test Plan: The tracks or the albums are still added to the playlist but much faster and without blocking the UI.

Reviewers: #elisa, astippich

Reviewed By: #elisa, astippich

Subscribers: astippich

Tags: #elisa

Maniphest Tasks: T7886

Differential Revision: https://phabricator.kde.org/D10451