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/TitleManagerDialog.cpp

@@ -30,7 +30,8 @@
 #include "ui_TitleManagerDialog.h"
 #include "ui_TitleImportDialog.h"
 
-using namespace Platform;
+using namespace melonDS;
+using namespace melonDS::Platform;
 
 std::unique_ptr<DSi_NAND::NANDImage> TitleManagerDialog::nand = nullptr;
 TitleManagerDialog* TitleManagerDialog::currentDlg = nullptr;