highgui: expose saveWindowParameters and loadWindowParameters to Python

Both functions persist/restore a window's size, position, flags,
trackbar values, and zoom/pan state (Qt highgui backend only), take
only a plain String argument, and were already fully documented -- but
were declared CV_EXPORTS instead of CV_EXPORTS_W, so they were
unreachable from Python.

Sibling Qt-only functions in the same header (displayOverlay,
displayStatusBar) already follow the CV_EXPORTS_W + runtime
"not implemented without Qt" convention this change follows.

Adds a Python test that exercises namedWindow + save/loadWindowParameters
and gracefully skips when no GUI backend or no Qt support is present,
consistent with how other environment-dependent tests in this suite
(e.g. CUDA) skip rather than fail.

Verified locally: built opencv_python3 from this branch (WITH_QT=OFF)
and confirmed cv2.saveWindowParameters / cv2.loadWindowParameters are
present on the built module and correctly raise the expected
"compiled without QT support" error, proving the binding reaches the
C++ layer end-to-end.

modules/highgui/misc/python/test/test_window_parameters.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python
+from __future__ import print_function
+
+import cv2 as cv
+
+from tests_common import NewOpenCVTests
+
+
+class window_parameters_test(NewOpenCVTests):
+
+    def test_save_load_window_parameters(self):
+        window_name = "test_save_load_window_parameters"
+        try:
+            cv.namedWindow(window_name, cv.WINDOW_NORMAL)
+        except cv.error:
+            self.skipTest("No GUI backend available in this build/environment")
+
+        try:
+            # Should not raise when the library is built with a backend that
+            # supports persisting window state (currently: Qt).
+            cv.saveWindowParameters(window_name)
+            cv.loadWindowParameters(window_name)
+        except cv.error as e:
+            # Expected on builds without Qt support (see NO_QT_ERR_MSG in window.cpp)
+            self.skipTest("saveWindowParameters/loadWindowParameters require the Qt "
+                           "highgui backend: " + str(e))
+        finally:
+            cv.destroyWindow(window_name)
+
+
+if __name__ == '__main__':
+    NewOpenCVTests.bootstrap()