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/widgets/fuel_gauge.rs

@@ -0,0 +1,32 @@
+use bevy::{color::palettes::css::*, prelude::*, ui::Val::*};
+
+use crate::game::consumables::{Fuel, FuelChanged};
+
+#[derive(Component)]
+#[require(Label)]
+pub struct FuelGauge(Entity);
+
+impl FuelGauge {
+  /// Default bundle for a machine's fuel gauge widget. Give it the ID of the
+  /// machine whose info it is displaying.
+  pub fn bundle(target: Entity) -> impl Bundle {
+    (
+      FuelGauge(target),
+      Node {
+        min_width: Px(20.0),
+        min_height: Px(10.0),
+        ..default()
+      },
+      BackgroundColor(GREEN.into()),
+      Text::new("Fuel: "),
+      children![TextSpan::new("<n>"),],
+    )
+  }
+
+  pub fn detect_fuel_change(event: Trigger<FuelChanged>) {
+    dbg!(format!(
+      "Detected fuel change on entity ID: {}",
+      event.target()
+    ));
+  }
+}