Implemented widget tree, gesture propagation and some more stuff:

- New naming rule: an "event" is triggered by a driver, a "gesture" is
  triggered by a tracker and delegated to the application by a widget.
- point down, move and up have been generalized to "events".
- Windows have been replaced by widgets, chich also support a tree structure.
- Gestures are propagated to parent widgets, events are delegated to child
  widgets. both propagation and delegation can be stopped by a handler.
- Gestures now save their originating event, events save their originating
  touch points.
- Test programs have been adapted to the new gesture binding style.