Port all selection effects to use PlasmaExtras.Highlight

I've found many hand-rolled implementations of selection and hover highlights throughout Plasma. As a result, most of them have subtle different semantics, visual states, and behaviors.

It's neither ideal nor even necessary, since we have PlasmaExtras.Highlight. Let's add any needed API to it and then port everything to use it, so Plasma applets' highlight effects can finally be not only consistent, but changeable from a central location.