Fix wxSlider::SetThumbLength() on wxMSW

It requires TBS_FIXEDLENGTH style.
Call InvalidateBestSize because the size of the control might change.
Call Layout in the widgets sample to adjust to the changed size.
Use GetThumbLength() instead of the arbitrary defined THUMB size.

samples/widgets/slider.cpp

@@ -543,6 +543,14 @@ void SliderWidgetsPage::DoSetThumbLen()
     }
 
     m_slider->SetThumbLength(len);
+
+    if ( m_slider->GetThumbLength() != len )
+    {
+        wxLogWarning(wxString::Format("Invalid thumb length in slider: %d",
+                                      m_slider->GetThumbLength()));
+    }
+
+    Layout();
 }
 
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