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/CheatsDialog.cpp

@@ -29,6 +29,7 @@
 #include "CheatsDialog.h"
 #include "ui_CheatsDialog.h"
 
+using namespace melonDS;
 using Platform::Log;
 using Platform::LogLevel;