Remove the Salva integration code from rapier + add a plugin system to the testbed.

src_testbed/plugin.rs

@@ -0,0 +1,12 @@
+use crate::testbed::PhysicsState;
+use kiss3d::window::Window;
+use na::Point3;
+
+pub trait TestbedPlugin {
+    fn init_graphics(&mut self, window: &mut Window, gen_color: &mut dyn FnMut() -> Point3<f32>);
+    fn clear_graphics(&mut self, window: &mut Window);
+    fn run_callbacks(&mut self, window: &mut Window, physics: &mut PhysicsState, t: f32);
+    fn step(&mut self, physics: &mut PhysicsState);
+    fn draw(&mut self);
+    fn profiling_string(&self) -> String;
+}