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_PCap.h

@@ -24,6 +24,7 @@
 namespace LAN_PCap
 {
 
+using namespace melonDS;
 struct AdapterData
 {
     char DeviceName[128];