keep pointer of track listener

Summary:
the metadata scanning for the file browser is currently done in the gui
thread. in preparation for moving this to the musiclistenersmanager, keep a
pointer to the trackslistener so that it can be reused

Test Plan: tests pass, music is still scanned correctly

Reviewers: mgallien

Reviewed By: mgallien

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D15078