Refactor propgrid sample

Simplify implementation by creating panel and sizer holding wxPG instance
only once in FormMain ctor.

samples/propgrid/propgrid.h

@@ -158,13 +158,12 @@ public:
 
 
     void CreateGrid( int style, int extraStyle );
+    void ReplaceGrid(int style, int extraStyle);
     void FinalizeFramePosition();
 
     // These are used in CreateGrid(), and in tests to compose
     // grids for testing purposes.
-    void InitPanel();
     void PopulateGrid();
-    void FinalizePanel( bool wasCreated = true );
 
     void PopulateWithStandardItems();
     void PopulateWithExamples();