use showMailAction form GrantleeHeaderStyle in formatter.

Summary:
I do see no way to change showMailAction - it is simply constant.
Additionally I do not understand how newmessageaction get into the
headerObject.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D13016