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/LAN_Socket.cpp

@@ -37,6 +37,7 @@
 	#include <time.h>
 #endif
 
+using namespace melonDS;
 
 namespace LAN_Socket
 {