makes data classes be implicitely shared: decrease memory usage

Summary:
use QSharedDataPointer to implictely share the data classes
(MusicAudioTrack, MusicAlbum and MusicAudioGenre)

A little reduction n memory usage is measurable (at least on my setup).

A further optimization could be to only create a track once in
DatabaseInterface.

CCBUG: 396840

Reviewers: astippich, januz, ognarb

Reviewed By: astippich

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D15456