- start_stroke()/finih_stroke() -> begin_stroke()/end_stroke()
- process_event_queue() -> process_stroke()
- GimpMotionBuffer::motion() -> GimpMotionBuffer::stroke()
- add GimpMotionBuffer::hover() and process_hover()
- remove push_event_history() and pop_event_queue() from API

The thing works like this:

- Motion events are continuously fed into the buffer using motion_event()
- begin_stroke()/end_stroke() correspond to BUTTON_PRESS/BUTTON_RELEASE,
  the period between them is a "stroke"
- If motion_event() returns TRUE, we request "stroke" signals by calling
  process_stroke() and "hover" signals by calling process_hover()