kfileitemmodel: let worker fetch mimetype

Méven Carrequested to merge
work/meven/request-mime-in-lister into master

Set KCoreDirLister::setRequestMimeTypeWhileListing by default. When supported this allows the kioworker to do mime-determination freeing the main-thread of this work.

This should help main-thread business in particular when GeneralSettings::hideXTrashFile is true.

Builds on top frameworks/kio!371 (merged)

cc @ahiemstra

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