egui support

- New Resource, EguiQueue: Utility to queue egui ui builders
- New Rendering System, EguiRenderer: Utilizes a modified egui_glium
  library to render egui UI
- Adjusted RenderingSystem trait and RenderingContext structure to
  provide event_loop and egui_queue.
- Various minor adjusments

platform/src/context.rs

@@ -1,6 +1,8 @@
+use std::{cell::RefCell, rc::Rc};
 use std::sync::{Arc, Mutex};
 use winit::event::Event;
 use raidillon_assets::ModelManagerRef;
+use raidillon_core::EguiQueue;
 
 #[derive(Clone)]
 pub struct PlatformContext {
@@ -10,6 +12,7 @@ pub struct PlatformContext {
     pub frame_height: f32,
     pub time_ctx: TimeContext,
     pub window: Arc<Mutex<winit::window::Window>>,
+    pub egui_queue: Rc<RefCell<EguiQueue>>,
 }
 
 #[derive(Clone)]