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.

src/generic/renderg.cpp

@@ -108,6 +108,8 @@ public:
 
     virtual wxSize GetCheckBoxSize(wxWindow *win) wxOVERRIDE;
 
+    virtual wxSize GetExpanderSize(wxWindow *win) wxOVERRIDE;
+
     virtual void DrawPushButton(wxWindow *win,
                                 wxDC& dc,
                                 const wxRect& rect,
@@ -719,6 +721,13 @@ wxSize wxRendererGeneric::GetCheckBoxSize(wxWindow *win)
     return win->FromDIP(wxSize(16, 16));
 }
 
+wxSize wxRendererGeneric::GetExpanderSize(wxWindow *win)
+{
+    wxCHECK_MSG( win, wxSize(0, 0), "Must have a valid window" );
+
+    return win->FromDIP(wxSize(16, 16));
+}
+
 void
 wxRendererGeneric::DrawPushButton(wxWindow *win,
                                   wxDC& dc,