frontend: input widgets: allow mouse/keyboard control

### Current behaviour

Dial and slider require clicking and dragging to interact with.

That's a valid interaction mechanism, but there are other options too, which could be nice to support.

### Expected or desired behaviour

Allow hovering over a variable widget and scrolling or pressing arrow keys to interact with it.

### Additional context

Raised by @rafaellehmkuhl in a meeting.

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.