Merge pull request #25354 from LaurentBerger:uiname

Add cv::currentUIFramework #25354

issue https://github.com/opencv/opencv/issues/25329

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [ ] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/highgui/test/test_gui.cpp

@@ -205,4 +205,23 @@ TEST(Highgui_GUI, trackbar)
     EXPECT_NO_THROW(destroyAllWindows());
 }
 
+
+TEST(Highgui_GUI, currentUIFramework)
+{
+    auto framework = currentUIFramework();
+    std::cout << "UI framework: \"" << framework << "\"" << std::endl;
+#if (!defined(ENABLE_PLUGINS) \
+        && !defined HAVE_GTK \
+        && !defined HAVE_QT \
+        && !defined HAVE_WIN32UI \
+        && !defined HAVE_COCOA \
+        && !defined HAVE_WAYLAND \
+    )
+    EXPECT_TRUE(framework.empty());
+#elif !defined(ENABLE_PLUGINS)
+    EXPECT_GT(framework.size(), 0);  // builtin backends
+#endif
+}
+
+
 }} // namespace