Split Win32 main window session state

src/platform_windows/windows_main_window_session.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <windows.h>
+
+namespace pp::platform::windows {
+
+[[nodiscard]] HWND& retained_main_window_handle_ref() noexcept;
+[[nodiscard]] wchar_t* retained_main_window_title_buffer() noexcept;
+[[nodiscard]] const wchar_t* retained_main_window_title() noexcept;
+[[nodiscard]] bool retained_main_window_sandboxed() noexcept;
+void set_retained_main_window_sandboxed(bool sandboxed) noexcept;
+
+}