[issue171] make all composer crypto operations async

[Marc Mutz]

New submission from Marc Mutz <marc at klaralvdalens-datakonsult.se>:

And provide a progress bar and cancel button, like in Kleopatra.

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messages: 825
nosy: marc
priority: wish
status: unread
title: make all composer crypto operations async
topic: KMail
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Aegypten issue tracker <aegypten-issues at intevation.de>
<https://intevation.de/roundup/aegypten/issue171>
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