- Implement WM_ENABLE handler to update cached control style when the

- Implement WM_ENABLE handler to update cached control style when the
  control is disabled.
- Move background drawing into WM_ERASEBKGND handler and draw the
  background differently when disabled.
- Change WM_PAINT handler to draw text with transparent background and
  with the correct colour for the style.
- Initialize selected field to -1 instead of 0 so that the control
  notices when field 0 is selected.