Properly restore "Device" combobox when the output model resets

Jakob Petsovitsrequested to merge
work/cherry-pick-cbdbfd43 into Plasma/6.0

ScreenSortProxyModel::setSourceModel(nullptr) will cause the ComboBox to set the currentIndex property to -1, which also kills its binding. We restore the binding and ensure that we're setting an output to a valid index within the newly reset model.

(cherry picked from commit cbdbfd43)

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