bug on gtwvg button

[JoseQuintas]

On pushbutton Destroy() do not remove image, causing memory usage and memory corruption after too many use.
Not sure if this is the best way, but solve my needs on Harbour 3.4

from:

METHOD wvgPushbutton:destroy()

   ::wvgWindow:destroy()

   RETURN Nil

to

METHOD wvgPushbutton:destroy()

   LOCAL oOldImage

   oOldImage := ::SendMessage( BM_SETIMAGE, WIN_IMAGE_ICON, Nil )
   IF oOldImage != Nil
      wvg_DeleteObject( oOldImage )
   ENDIF
   ::wvgWindow:destroy()

   RETURN Nil