frontend: very generic indicator: allow showing progress/state

[ES-Alexander]

Problem to solve

Variables being displayed by an indicator often have some kind of logical value range over which they are healthy or partially complete (e.g. temperature, tether spool-out length, battery, etc), but we currently can't show those without a dedicated widget to do so.

Desired approach

1. Support colour ranges with threshold/gradient-defining values
   • partially suggested in VeryGenericIndicator UI and processing improvements #613
2. Support circular progress bars around the icon
   ...

Additional context

• Some inspiration

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.