fix support for multiple tracks in same album with same title

Summary:
in case some tracks have same title but differing track or disc number,
this patch allow inserting them in database without errors

Test Plan: new test is OK, no regressions

Reviewers: Elisa, ngraham

Reviewed By: ngraham

Subscribers: ngraham

Differential Revision: https://phabricator.kde.org/D21166