Fix crash when switching signed/encrypted emails very fast

Kleo::KeyCache internally uses a QEventLoop to block until it
finishes initializing. The initialization happens the first
time the user opens a signed or encrypted email. If they switch
to another email before the KeyCache is done initializing it
leads to a crash in MessageViewer, since message pointers change
while MessageViewer is still "blocked" in OTP.

To avoid the crash, and running a nested event loop in general,
this change introduces KeyCacheMemento, which allows to wait for
the KeyCache to initialize asynchronously before the MessagePart
queries it.

(cherry picked from commit 624cee41
from 23.04 branch, because I forgot to merge it to master, so it did
not merge it to 23.08)

BUGS: 463083