move MediaPlaylist to C++
Summary:
first step of optimization after the introduction of the dedicated proxy model.
already cleans up a lot of the interface and makes the Qml code easier. Performance does not seem to change much.
no functional change intended. all tests pass
Reviewers: Elisa, mgallien
Reviewed By: Elisa, mgallien
Subscribers: mgallien
Tags: Elisa
Differential Revision: https://phabricator.kde.org/D10271