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