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/system.rs

@@ -0,0 +1,42 @@
+use std::cell::RefCell;
+use std::rc::Rc;
+use raidillon_core::scene::Scene;
+use indexmap::IndexMap;
+use winit::event::Event;
+use raidillon_core::context::PlatformContext;
+use raidillon_core::DebugUIBuffer;
+
+pub struct SystemContext<'a> {
+    // TODO: time delta etc.
+    pub scene: &'a mut Scene,
+    pub platform_context: PlatformContext,
+    pub debug_ui_buffer: Rc<RefCell<DebugUIBuffer>>,
+}
+
+pub trait System {
+    /// Initialize the system.
+    fn initialize(&mut self);
+    /// Spawn the first entities of the world.
+    fn load_world(&mut self, ctx: &mut SystemContext);
+    fn update(&mut self, ctx: &mut SystemContext);
+}
+
+pub type SystemID = &'static str;
+pub struct SystemManager {
+    pub systems: IndexMap<SystemID, Box<dyn System>>,
+}
+
+impl SystemManager {
+    pub fn new() -> Self {
+        let systems = IndexMap::default();
+        Self { systems }
+    }
+
+    pub fn add_system(&mut self, id: SystemID, system: Box<dyn System>) {
+        self.systems.insert(id, system);
+    }
+
+    pub fn remove_system(&mut self, id: SystemID) {
+        self.systems.shift_remove(&id);
+    }
+}