[applet] Port to checkbox with internally-defined icon property

With https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/46,
checkboxes will be able to have an icon property defined internally, no
need for a separate SVG. So if and when that lands, we can use that and
discard the bespoke SwitchButton component.

This is an alternative to
plasma/plasma-nm!13.