Speed up opening the folder properties dialog in kmail.

An IMAP mail check schedules a large number of tasks.
Tasks that the user is waiting for, such as ItemFetch (reading one mail)
or SyncCollectionAttributes (as called by the folder properties dialog)
should have priority over that.

There might be more types of tasks that the user could potentially be waiting for.
Maybe we could even blacklist instead of whitelist, i.e. put folder-sync tasks
in a low-prio queue, while the default queue would be above it. Not sure.

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