Use correct expander size in wxDataViewCtrl under MSW

Add wxRendererNative::GetExpanderSize(), implement it using the
appropriate theme element and use it instead of hardcoding the expander
size to 3 character widths.

Closes https://github.com/wxWidgets/wxWidgets/pull/1431

Closes #18449.

include/wx/renderer.h

@@ -253,6 +253,9 @@ public:
     // Returns the default size of a check box.
     virtual wxSize GetCheckBoxSize(wxWindow *win) = 0;
 
+    // Returns the default size of a expander.
+    virtual wxSize GetExpanderSize(wxWindow *win) = 0;
+
     // draw blank button
     //
     // flags may use wxCONTROL_PRESSED, wxCONTROL_CURRENT and wxCONTROL_ISDEFAULT
@@ -479,6 +482,9 @@ public:
     virtual wxSize GetCheckBoxSize(wxWindow *win) wxOVERRIDE
         { return m_rendererNative.GetCheckBoxSize(win); }
 
+   virtual wxSize GetExpanderSize(wxWindow *win) wxOVERRIDE
+        { return m_rendererNative.GetExpanderSize(win); }
+
     virtual void DrawPushButton(wxWindow *win,
                                 wxDC& dc,
                                 const wxRect& rect,