[sddm] Fix collapsed width of a volume/brightness OSD on a lock screen
Amends c067d498. Having `preferredWidth: 1` set on an only item that also stretches with fillWidth AND without top-to-bottom sizing (fixed size imposed by parent component) doesn't really make sense. It naturally increased row layout's implicitWidth by 1, and that value were used as an input to the final width expression (bottom-up sizing). This patch fixes the sizing model to be strictly implicit bottom-up and explicit top-down, and so the slider no longer collapses.