Initial work on rollback and refactor

src/input/control.rs

@@ -0,0 +1,12 @@
+//! Which entity is driving which. Kept separate from the input pipeline
+//! itself: this is a relationship between entities, not a source of input.
+
+use bevy::prelude::*;
+
+#[derive(Component, Debug)]
+#[relationship(relationship_target = ControlledBy)]
+pub struct Controls(pub Entity);
+
+#[derive(Component, Debug)]
+#[relationship_target(relationship = Controls)]
+pub struct ControlledBy(Entity);