Fix generation of extraneous wxEVT_SLIDER events in wxMSW

Don't send the event when it's redundant, i.e. doesn't really notify
about the change in the slider value.

Also add a test case for wxEVT_SLIDER and show these events in the
widgets sample.

Closes https://github.com/wxWidgets/wxWidgets/pull/2080

Closes #18929.

tests/controls/slidertest.cpp

@@ -35,6 +35,7 @@ private:
 #ifndef __WXOSX__
         WXUISIM_TEST( PageUpDown );
         WXUISIM_TEST( LineUpDown );
+        WXUISIM_TEST( EvtSlider );
         WXUISIM_TEST( LinePageSize );
 #endif
         CPPUNIT_TEST( Value );
@@ -47,6 +48,7 @@ private:
 
     void PageUpDown();
     void LineUpDown();
+    void EvtSlider();
     void LinePageSize();
     void Value();
     void Range();
@@ -125,6 +127,24 @@ void SliderTestCase::LineUpDown()
 #endif
 }
 
+void SliderTestCase::EvtSlider()
+{
+#if wxUSE_UIACTIONSIMULATOR
+    EventCounter slider(m_slider, wxEVT_SLIDER);
+
+    wxUIActionSimulator sim;
+    wxYield();
+    m_slider->SetFocus();
+
+    sim.Char(WXK_UP);
+    sim.Char(WXK_DOWN);
+
+    wxYield();
+
+    CPPUNIT_ASSERT_EQUAL(2, slider.GetCount());
+#endif
+}
+
 void SliderTestCase::LinePageSize()
 {
 #if wxUSE_UIACTIONSIMULATOR