lazy initialize as much as possible
Summary:
lazy initialize the standard actions
lazy initialize all the models and the MusicListenersManager after start
Test Plan:
everything seems to work like it used to work
the interface is shown earlier but with a busy indicator
Reviewers: Elisa, astippich
Reviewed By: Elisa, astippich
Subscribers: astippich
Tags: Elisa
Maniphest Tasks: T7887
Differential Revision: https://phabricator.kde.org/D10402