Refactor NDS and DSi to be objects (#1893)

* First crack at refactoring NDS and DSi into objects

- Remove all global/`static` variables in `NDS` and related classes
- Rely more on virtual dispatch when we need to pick methods at runtime
- Pass `NDS&` or `DSi&` to its constituent components where necessary
- Introduce some headers or move some definitions to break `#include` cycles

* Refactor the frontend to accommodate the core's changes

* Move up `SchedList`'s declaration

- Move it to before the components are initialized so the `map`s inside are initialized
- Fields in C++ are initialized in the order they're declared

* Fix a crash when allocating memory

* Fix JIT-free builds

* Fix GDB-free builds

* Fix Linux builds

- Explicitly qualify some member types in NDS, since they share the same name as their classes

* Remove an unnecessary template argument

- This was causing the build to fail on macOS

* Fix ARM and Android builds

* Rename `Constants.h` to `MemConstants.h`

* Add `NDS::IsRunning()`

* Use an `#include` guard instead of `#pragma once`

src/frontend/qt_sdl/main.h

@@ -40,6 +40,11 @@
 #include "FrontendUtil.h"
 #include "duckstation/gl/context.h"
 
+namespace melonDS
+{
+class NDS;
+}
+
 class EmuThread : public QThread
 {
     Q_OBJECT
@@ -67,7 +72,8 @@ public:
     QMutex FrontBufferLock;
 
     void updateScreenSettings(bool filter, const WindowInfo& windowInfo, int numScreens, int* screenKind, float* screenMatrix);
-
+    void RecreateConsole();
+    std::unique_ptr<melonDS::NDS> NDS; // TODO: Proper encapsulation and synchronization
 signals:
     void windowUpdate();
     void windowTitleChange(QString title);
@@ -90,6 +96,7 @@ signals:
     void syncVolumeLevel();
 
 private:
+    std::unique_ptr<melonDS::NDS> CreateConsole();
     void drawScreenGL();
     void initOpenGL();
     void deinitOpenGL();