Speed up NotificationSubscriber::acceptsItemNotification

Hotspot spotted a lot of time spent resolving mimetype aliases
while processing notifications
http://www.davidfaure.fr/2020/qmime_alias.png

We might as well resolve to canonical aliases upfront,
setting up subscriptions happens a lot less than actual notifications.
1 job for master in 32 minutes and 19 seconds (queued for 3 seconds)
Status Job ID Name Coverage
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passed #61273
linux kf5-qt5 qt5.12

00:32:19

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