Add a size selection choice to the "artprov" sample

This will allow to test support for larger icons when it is added in the
upcoming commits.

samples/artprov/artbrows.h

@@ -28,12 +28,19 @@ public:
 
 private:
     void OnSelectItem(wxListEvent &event);
+    void OnChangeSize(wxCommandEvent &event);
     void OnChooseClient(wxCommandEvent &event);
 
+    wxSize GetSelectedBitmapSize() const;
+
     wxListCtrl *m_list;
     wxStaticBitmap *m_canvas;
     wxStaticText *m_text;
     wxString m_client;
+    wxChoice *m_sizes;
+    wxString m_current_artid;
+
+    static int m_bitmap_sizes[7];
 
     wxDECLARE_EVENT_TABLE();
 };