Send wxEVT_SYS_COLOUR_CHANGED on dark/light theme change in wxMSW

wxOSX already does it, so do it in wxMSW too for consistency and because
it seems useful to be able to react to the same event in all cases.

Update the drawing sample to refresh its system colours scheme whenever
the theme is changed.

samples/drawing/drawing.cpp

@@ -524,6 +524,13 @@ MyCanvas::MyCanvas(MyFrame *parent)
     m_useBuffer = false;
     m_showBBox = false;
     m_sizeDIP = wxSize(0, 0);
+
+    Bind(wxEVT_SYS_COLOUR_CHANGED, [this](wxSysColourChangedEvent& event) {
+        event.Skip();
+
+        if ( m_show == File_ShowSystemColours )
+            Refresh();
+    });
 }
 
 void MyCanvas::DrawTestBrushes(wxDC& dc)