Add encryptionDetails to the messageviewer.

Summary:
Sometimes you want to see for what keys the message was encrypted. One
example is, if you awnser and need first to download the corresponding
key.

Test Plan:
open encrypted message

not decrypted:
{F4398841}

encrypted with light theme:
{F5448201}

encrypted with light theme (expanded info):
{F5448202}

encrypted with light theme (failed to encrypt):
{F5448203}

encrypted with dark theme:
{F5448206}

encrypted with dark theme (expanded info):
{F5448205}

encrypted with dark theme (failed to encrypt):
{F5448207}

Reviewers: vkrause, dvratil, colomar

Subscribers: #vdg, jensreuterberg, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8110