Revert "[sddm-theme] Clean up footer buttons code"

This partially reverts commit d2bb4807.

Something unhealthy is going on with that lastIndex that prevents it from functioning normally when currentIndex is bound to it from the beginning. Maybe it just needs an on*Changed() signal, so that the instantiator has already created its objects by that time? Session button is different from keyboard layouts button which uses pre-baked QList as its model.