Sort enabled outputs first

We want the combobox and stackview to show the most relevant outputs
first.

As indexes are passed between here and the top view we move sorting
lower in the stack.

This also saves QML from having to deal with the model object changing.
From a QML POV it'll now be the same object resetting.

> visible: kcm.outputModel.rowCount() > 1

Is ported as this is unsafe. We're calling an invokable as a property so
it won't update. A new explicit property is introduced on the KCM.


(cherry picked from commit 5cf9c326)