Extract Win32 runtime state storage

src/platform_windows/windows_runtime_state.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <memory>
+
+class App;
+class WacomTablet;
+
+namespace pp::platform::windows {
+
+[[nodiscard]] std::unique_ptr<App>& retained_owned_app() noexcept;
+[[nodiscard]] WacomTablet& retained_wacom_tablet() noexcept;
+
+}