[applet] Refactor bluetooth state properties

Remove the logic.js helper file and prefer to set the plasmoid properties declaratively. Along with some javascript modernizing.

No visual or behavioural changes.

I've been doing this in an attempt to fix https://bugs.kde.org/show_bug.cgi?id=438610. Well, it doesn't fix it, but now we're certain there's a different cause than imperative code not updating the bindings.