KMail: don't move the mainwindow to the current desktop when invoking a composer

Summary:
Bringing up a composer or a viewer from the cmdline doesn't require
moving the mainwindow to the current desktop (which has annoyed me for a
long time).

This code assumes that kmail <any args> is always about bringing up a
composer or a viewer window; I didn't find another case in kmkernel's
handleCommandLine.

Test Plan: kmail faure@kde.org from another desktop.

Reviewers: dvratil, mlaurent

Subscribers: KDE PIM

Differential Revision: https://phabricator.kde.org/D13301