HostDisplay: Move most backend logic to FrontendCommon

src/duckstation-qt/qtdisplaywidget.h

@@ -1,6 +1,8 @@
 #pragma once
 #include "common/types.h"
+#include "common/window_info.h"
 #include <QtWidgets/QWidget>
+#include <optional>
 
 class QtDisplayWidget final : public QWidget
 {
@@ -16,6 +18,8 @@ public:
   int scaledWindowHeight() const;
   qreal devicePixelRatioFromScreen() const;
 
+  std::optional<WindowInfo> getWindowInfo() const;
+
 Q_SIGNALS:
   void windowResizedEvent(int width, int height);
   void windowRestoredEvent();