[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