PimUniqueApplication: make newInstance virtual.

Summary:
This will be useful for kmail to be able to control whether
the mainwindow moves to the current desktop, depending on
the way the second kmail is invoked.

Test Plan: compiles

Reviewers: dvratil, mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D13300