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.cpp

@@ -0,0 +1,13 @@
+#include "copium/event/ViewportResize.h"
+
+namespace Copium
+{
+  ViewportResize::ViewportResize(const BoundingBox& viewport)
+    : Event(EventType::ViewportResize), viewport{viewport}
+  {}
+
+  const BoundingBox& ViewportResize::GetViewport() const
+  {
+    return viewport;
+  }
+}