Moved implementation to dedicated class 'CImguiSystem' with the following changes:
* Added mutex for ImGui_ImplWin32_WndProcHandler(), the window proc is ran from a different thread than CImguiSystem::SampleFrame() is being called from. Needs a mutex as both functions interact with the input queue list.
* Added init stages for debugging and preventing the system from running certain parts of the code if a certain stage hasn't been reached yet.
* NULL out ImGuiContext::ConfigNavWindowingKeyNext , we don't need any nav window shortcuts as users don't need then in-game, and it also interferes with certain key binds some users have.
* CImguiSystem::SampleFrame() (previously ImguiSystem_SampleFrame()), is now being called from CMaterialSystem::SwapBuffers(), as that function is called from the main thread while also getting updated during level loads/etc, so instead of switching between 2 static buffers for as long as the levels are being loaded, we imgui updates properly too and directly swap afterwards.
* Removed hack in CGame::WindowProc() where we would only run windowproc if we aren't shutting the game down. This was needed as there was a chance of a crash at some rare cases, but this was simply due to a missing mutex which has been addressed and fixed as of this commit.
* Lock mutex during init and shutdown.
ImGui drawing code now takes place in the main thread, a snapshot of the render data is created in CMaterialSystem::SwapBuffers(), and is being rendered in the render thread right before SpinPresent().
The reason why this was necessary, is because ConVar::GetString() isn't thread safe if its not marked FCVAR_MATERIAL_SYSTEM_THREAD or FCVAR_ACCESSIBLE_FROM_THREADS, and we used it for the console suggestions window, which iterates over every ConVar, accessible from threads or not.
