Add docking of main viewport

- Change ImGui branch to docking
- Rework code to handle viewport resizes instead of window resize

CopiumEngine/src/copium/event/ViewportResize.h

@@ -0,0 +1,17 @@
+#pragma once
+
+#include "copium/event/Event.h"
+#include "copium/event/EventType.h"
+#include "copium/util/BoundingBox.h"
+
+namespace Copium
+{
+  class ViewportResize : public Event
+  {
+    BoundingBox viewport;
+  public:
+    ViewportResize(const BoundingBox& viewport);
+
+    const BoundingBox& GetViewport() const;
+  };
+}