Move all core types into namespaces (#1886)

* Reorganize namespaces

- Most types are now moved into the `melonDS` namespace
- Only good chance to do this for a while, since a big refactor is next

* Fix the build

src/frontend/qt_sdl/SaveManager.cpp

@@ -22,7 +22,8 @@
 #include "SaveManager.h"
 #include "Platform.h"
 
-using namespace Platform;
+using namespace melonDS;
+using namespace melonDS::Platform;
 
 SaveManager::SaveManager(const std::string& path) : QThread()
 {