Turn Attachment into an implicitly shared value type

Authored by vkrause on Jul 2 2019, 6:01 PM.

Description

Turn Attachment into an implicitly shared value type

Summary:
Avoids the extra allocation, and makes this easier to directly expose to
QML/Grantlee/etc.

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D22244