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/Config.cpp

@@ -26,6 +26,7 @@
 
 namespace Config
 {
+using namespace melonDS;
 
 int KeyMapping[12];
 int JoyMapping[12];