Extract brush panel UI, app startup shell, and Win32 runtime shell

src/platform_windows/windows_runtime_shell.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include "platform_windows/windows_bootstrap_helpers.h"
+#include "platform_windows/windows_splash.h"
+
+namespace pp::platform::windows {
+
+void run_main_window_runtime(const MainWindowStartupState& startup, bool start_in_vr, SplashScreen& splash);
+
+}