todo

基于SDL实现的GUI

- DOCUMENTATION IS NEEDED! 
- Add more widgets:
  Slider
  ListBox
  TextArea
  TreeList
- ESC causes GUI to exit. This needs to be configurable.
- editting keys in TextEntry: home/end/arrows
- TextEntry needs a cursor (caret? whatever)
- make the demo install to /usr/share, and add an RPM
  for the demo. this should make it easier to try out the
  library.
- background tile function does not like negative offsets.