Proof-of-concept for fuel change event handling

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.

src/game/mod.rs

@@ -34,4 +34,7 @@ pub mod consumables {
 
   #[derive(Component)]
   pub struct Fuel(u32);
+
+  #[derive(Event)]
+  pub struct FuelChanged;
 }