height (fixes overlap of image and text in IE4).
- Handle return codes from TBN_DROPDOWN (allows a "dropdown" to be
  treated like a regular button).
- Implement TBN_BEGINDRAG and TBN_ENDDRAG in WM_LBUTTON{DOWN|UP} (not
  necessary but done by native).
- Implement NM_RELEASEDCAPTURE and NM_CLICK.