makes AllAlbumsModel use a thread for long operations
Summary:
optimize the link from DatabaseInterface to AllAlbumsModel
use a thread for operations taking a long time in AllAlbumsModel
send multiple new albums in one signal
avoid sending modified album signals when we are modifying newly created
albums
make automatic tests of AllAlbumsModel more reliable
the other models will receive the same kind of modifications but I would
prefer to keep this review small
Test Plan: automatic tests are OK and the application feels fast
Reviewers: Elisa, astippich
Reviewed By: Elisa, astippich
Subscribers: astippich
Tags: Elisa
Maniphest Tasks: T7886
Differential Revision: https://phabricator.kde.org/D10374