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)

src/msw/window.cpp

@@ -5106,6 +5106,13 @@ bool wxWindowMSW::HandleCaptureChanged(WXHWND hWndGainedCapture)
 
 bool wxWindowMSW::HandleSettingChange(WXWPARAM wParam, WXLPARAM lParam)
 {
+    // Check for the special case of changing the system light/dark mode.
+    if ( lParam && wxStrcmp((TCHAR*)lParam, wxT("ImmersiveColorSet")) == 0 )
+    {
+        // Forward to the existing function generating an event for this.
+        HandleSysColorChange();
+    }
+
     // despite MSDN saying "(This message cannot be sent directly to a window.)"
     // we need to send this to child windows (it is only sent to top-level
     // windows) so {list,tree}ctrls can adjust their font size if necessary