button; use plain mouse capture instead like native.
- Return only HTTRANSPARENT/HTCLIENT from WM_NCHITTEST and remove
  associated hacks of WM_SETCURSOR, WM_NCLBUTTONDOWN and
  WM_NCLBUTTONUP.
- Refactor state change code so that state changes don't happen as
  side effects from messages such as WM_NCCALCSIZE, instead only from
  user input messages like WM_MOUSEMOVE.