UI: Self-formatting number label widget

A simple Label subclass with `value` and `valuePercent` bind points that then formats incoming values according to some strategy chosen at construction.

As a demo the infoview fuel gauge has been updated to use it. Don't think too much about whether UI.InfoGauge is right, I'm not happy with it and it will likely be changed further.

I suppose down the track some more generic formatting label (or even whole framework) might be needed. I don't think we're going to know properly until the new UI is pushed through to more places.