common improvements (#366)

- std container replacements for vector, unordered_map, unordered_set
  - unordered_map and unordered_set replaced with ska::flat_hash_map
  - vector replacement is still std::
- unordered_map performance test (std:: vs ska::)
- backlog improvements
- wiArchive improvements
- editor: added option to dump scene to C++ header file
- common improvements, refactors

Example_ImGui/Example_ImGui.cpp

@@ -10,8 +10,6 @@
 #include "ImGui/imgui_impl_sdl.h"
 #endif
 
-#include <string>
-#include <sstream>
 #include <fstream>
 #include <thread>