Assorted code quality improvements to ScrollView and IconLabel
ScrollView: Use standard implicit sizing expressions
PlasmaExtras: Clean up some QML/JS
IconLabel: Remove unused properties
They were added as a convenience/shortcuts, but later were replaced with bindings on grid's flow and child items' visibility.
IconLabel: Replace unstyled T.Label with base Text component
We are not using any of the Label's features: neither QQC2 style or background or insets. So there's practically no reason for it to be T.Label specifically.