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/RAMInfoDialog.cpp

@@ -21,6 +21,7 @@
 
 #include "main.h"
 
+using namespace melonDS;
 extern EmuThread* emuThread;
 
 s32 GetMainRAMValue(const u32& addr, const ramInfo_ByteType& byteType)