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.

BUGS: 463083