Port to upstream components where possible
Basically: - PlasmaExtras.Heading -> Kirigami.Heading - PlasmaComponents3.Label -> QQC2.label We can do this because these components only existed in the past to use special Plasma colors, but no visual styling. And the color stuff is obsolete because Kirigami colors work correctly in Plasma now. All widgets were tested with a dark Plasma theme and a light color scheme to ensure no regressions.