Implement basic gun controller

The spacebar fires the gun, bullets are spawned on top of the ship with
a velocity that sends them forward (relative to the ship).

I still need a despawn mechanism, and a fire rate control. To despawn
things, I'm already planning a `Lifetime` component. For rate of fire,
an additional component will be created and attached to the player ship.

src/physics.rs

@@ -5,7 +5,7 @@ use crate::WorldSize;
 
 use bevy::prelude::*;
 
-#[derive(Component)]
+#[derive(Clone, Component)]
 pub(crate) struct Velocity(pub(crate) bevy::math::Vec2);
 
 #[derive(Component)]