Add an identifier icon in the linked accounts inbox

[johanftg]

It would be really helpful to have an icon for each email in the combined inbox that shows the corresponding account. This feature would make it easier to identify similar emails from different accounts without having to open each one to check the origin.

[bkaankose]

Account nickname indicators for linked inboxes are implemented for versions higher than 2.0.25.0.

Closing as fixed.