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/ROMManager.h

@@ -31,6 +31,7 @@
 namespace ROMManager
 {
 
+using namespace melonDS;
 extern SaveManager* NDSSave;
 extern SaveManager* GBASave;
 extern std::unique_ptr<SaveManager> FirmwareSave;