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

core/src/engine.rs

@@ -1,5 +1,3 @@
-use std::cell::RefCell;
-use std::rc::Rc;
 use crate::DebugUIBuffer;
 use crate::scene::Scene;
 
@@ -14,5 +12,4 @@ pub trait EngineTrait {
     fn current_scene(&self) -> &Scene;
     fn get_debug_ui_buffer(&self) -> &DebugUIBuffer;
     fn reset_debug_ui_buffer(&mut self);
-    // fn scene_and_debug_ui_buffer_mut(&mut self) -> (&mut Self::Scene, &DebugUIBuffer);
 }