Applet code modernization
- Port away from Kirigami usage for units. Use PlasmaCore.Units instead
- Port away from many usage of Plasma Components 2 to Plasma Components 3
- Port away from ScrollArea to QQC2.ScrollView
- Port away from PageStack to QQC2.StackView
- Add tooltips in a few places
- Use PlasmaExtras.PlasmoidHeading for the heading.
- Hide navigation back button when not needed
