wip: Changes of the week

- Move engine to a different crate
- Add engine trait
- Refactor the rest of the codebase to work with these changes
- Add debug ui buffer, use it to finish imgui support

engine/src/lib.rs

@@ -0,0 +1,4 @@
+pub mod engine;
+pub mod system;
+
+pub use crate::engine::Engine;