[rcore] [SDL2] Fix show, hide, focus and unfocus window/flags states

[asdqwe]

Fixes show, hide, focus and unfocus window/flags states on PLATFORM_DESKTOP_SDL, and IsWindowHidden() (ref) and IsWindowFocused() (ref) that were INOP on PLATFORM_DESKTOP_SDL because the platform was missing event handling for SDL_WINDOWEVENT_HIDDEN, SDL_WINDOWEVENT_SHOWN, SDL_WINDOWEVENT_FOCUS_GAINED and SDL_WINDOWEVENT_FOCUS_LOST (ref).

This PR was tested with SDL2.30.10 on Linux Mint 22.0. SDL3 wasn't tested but should work since these changes share the same events and the handling is agnostic.

The PR can be tested with:

#include "raylib.h"
int main(void) {
    InitWindow(800, 450, "test");
    SetTargetFPS(60);

    double interval = 2.0;

    while (!WindowShouldClose()) {

        // Show and Hide the window every two seconds:
        if (GetTime() > interval) {
            if (IsWindowHidden()) {
                ClearWindowState(FLAG_WINDOW_HIDDEN);
            } else {
                SetWindowState(FLAG_WINDOW_HIDDEN);
            }
            interval += 2.0;
        }

        TraceLog(LOG_INFO, "Hidden %i.  Focused %i.", IsWindowHidden(), IsWindowFocused());

        BeginDrawing();
        ClearBackground(RAYWHITE);
        EndDrawing();
     }
    CloseWindow();
    return 0;
}