[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 !4 (closed)

Please do not merge until and unless https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/46 is merged.