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/EmuSettingsDialog.cpp

@@ -29,7 +29,8 @@
 #include "EmuSettingsDialog.h"
 #include "ui_EmuSettingsDialog.h"
 
-using namespace Platform;
+using namespace melonDS::Platform;
+using namespace melonDS;
 
 EmuSettingsDialog* EmuSettingsDialog::currentDlg = nullptr;