Added code for erasing the small square between scrollbars.

  Added code for erasing the small square between scrollbars.
  Corrected window size if scrollbars is removed/added.
  Reduce flicker in scrollbars.
  Prevent scrollbars from jumping back to original position
    if the mouse just barely left the scrollbar.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@14872 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775