Upgrade serialized data format to v2 ("BB"), integrate new handlers

- Fix and upgrade BNF to v2 (magic: "BB").
  . simplify events, remove unnecessary/unused data,
  . add cursor positions to button-down and button-up events (fix #24).
- Add mouse coordinates to button events (fix #24).
- Remove client coordinates and unnecessary (redundant) movement data
  from mouse move events.
- Change debugStateHandler to a common stateHandler and extend emitted
  data with window positions and various window dimensions (inner,
  outer) (fix #23).
- Add and integrate window position handler that triggers on window
  position changes.
- Add and integrate visibility change handler (tab focus / blur) (fix
  #22 -- mostly by Olivér).
- Add and integrate window unload handler (fix #21 -- mostly by Olivér).
- Move marker trigger to handler.
- Add configurable handler start() through argument passing.
- Fix e2e test accordingly.