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/CameraManager.cpp

@@ -21,6 +21,7 @@
 #include "CameraManager.h"
 #include "Config.h"
 
+using namespace melonDS;
 
 #if QT_VERSION >= 0x060000