Make wxProgressDialog::Fit() work in native MSW version

This method is supposed to adjust the dialog size to its contents and
while the dialog increases automatically when using native
implementation under MSW, it doesn't shrink back on its own and so it's
still useful to allow Fit() to do it.

Update the sample to test Fit() too.

include/wx/msw/progdlg.h

@@ -44,6 +44,7 @@ public:
     virtual void SetIcons(const wxIconBundle& icons) wxOVERRIDE;
     virtual void DoMoveWindow(int x, int y, int width, int height) wxOVERRIDE;
     virtual void DoGetPosition(int *x, int *y) const wxOVERRIDE;
+    virtual void Fit() wxOVERRIDE;
 
     virtual bool Show( bool show = true ) wxOVERRIDE;