[KMail] Add an option to start in tray
Summary: Adds an option to launch Kmail minimized to tray
Reviewers: KDE PIM: KMail, mlaurent
Reviewed By: mlaurent
Subscribers: lbeltrame, dvratil, kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D19189