kfileitemmodel: let worker fetch mimetype
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