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/ROMManager.cpp

@@ -51,7 +51,8 @@ using std::string;
 using std::tie;
 using std::unique_ptr;
 using std::wstring_convert;
-using namespace Platform;
+using namespace melonDS;
+using namespace melonDS::Platform;
 
 namespace ROMManager
 {