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/PathSettingsDialog.cpp

@@ -27,6 +27,8 @@
 #include "PathSettingsDialog.h"
 #include "ui_PathSettingsDialog.h"
 
+using namespace melonDS::Platform;
+namespace Platform = melonDS::Platform;
 
 PathSettingsDialog* PathSettingsDialog::currentDlg = nullptr;