Add support for modal windows to more platforms

- Adds support for modal windows to Win32, Mac, and Haiku, and enhances functionality on Wayland and X11, which previous set only the parent window, but not the modal state.
- Windows can be declared modal at creation time, and the modal state can be toggled at any time via SDL_SetWindowModalFor() (tested with UE5 through sdl2-compat).
- Allows dynamic unparenting/reparenting of windows.
- Includes a modal window test.

src/video/wayland/SDL_waylandvideo.h

@@ -80,6 +80,7 @@ struct SDL_VideoData
     struct wp_fractional_scale_manager_v1 *fractional_scale_manager;
     struct zwp_input_timestamps_manager_v1 *input_timestamps_manager;
     struct zxdg_exporter_v2 *zxdg_exporter_v2;
+    struct xdg_wm_dialog_v1 *xdg_wm_dialog_v1;
     struct kde_output_order_v1 *kde_output_order;
 
     struct xkb_context *xkb_context;