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/egui_queue.rs

@@ -0,0 +1,22 @@
+pub struct EguiQueue {
+    pub queue_vec: Vec<Box<dyn FnOnce(&egui::Context) + Send>>,
+}
+
+impl EguiQueue {
+    pub fn new() -> Self {
+        Self { queue_vec: Vec::new() }
+    }
+
+    pub fn queue(&mut self, func: impl FnOnce(&egui::Context) + Send + 'static) {
+        self.queue_vec.push(Box::new(func));
+    }
+
+    pub fn clear(&mut self) {
+        self.queue_vec.clear()
+    }
+
+    pub fn run(&mut self, ctx: &egui::Context) {
+        self.queue_vec.drain(..).for_each(|func| func(ctx));
+        self.clear();
+    }
+}