FillRect and BF_MIDDLE on _FLAT toolbars.)
- Issue FIXMEs for unsupported styles.
- If the "hidden" state has changed then recalc the buttons.
- Support TBSTYLE_CUSTOMERASE (used by IE4) ans issue some of the
  necessary NM_CUSTOMDRAW notifies.
- Pass the WM_ERASEBKGND to parent if the toolbar is "transparent".
- Invalidate the area 1 bigger than the button rect on a WM_MOUSELEAVE
  so that the edge is "erased" by the parent.
- Support the PGN_CALCSIZE notify since a toolbar can be a child of the
  Pager control.