Bind Win32 app shell and move more legacy platform state

src/platform_windows/windows_runtime_shell.h

@@ -3,12 +3,15 @@
 #include "platform_windows/windows_bootstrap_helpers.h"
 #include "platform_windows/windows_splash.h"
 
+class App;
 class AppRuntime;
 
 namespace pp::platform::windows {
 
 int run_main_application(int argc, char** argv);
 void run_main_window_runtime(const MainWindowStartupState& startup, bool start_in_vr, SplashScreen& splash);
+void bind_app(App* app) noexcept;
+[[nodiscard]] App* bound_app() noexcept;
 void bind_runtime(AppRuntime* runtime) noexcept;
 [[nodiscard]] AppRuntime* bound_runtime() noexcept;