There is an on-add hook that can be used to maintain invariants. I'll
use it to ensure the label text always gets the current fuel value upon
spawning the widget, regardless of whether or not the caller remembers
to set it up themselves.
This does a linear search through all existing FuelGauges, but we're not
expecting that to be a problem. There should only be zero or one of them
on screen at any moment.
It also doesn't work because the rest of the game state isn't correct...
Added a `FuelChanged` event which is to be used to trigger Observers
watching for a machine's fuel level change.
I've written a slapdash observer to emit these events when the cutting
machine's big red button is pressed. That observer *must* be replaced
because of how it feeds the machine ID into the UI (it only works when
there is exactly 1 CuttingMachine, which won't be the case).
The dummy machines are missing their machine components. I've added the
`CuttingMachine` component to the cutting machine just to test the event
passing.
