swirl zoom support

[M4rtinK]

• swirl zoom for zooming the currently visible page
• similar in effect to pinch-zoom, but usable on non-multitouch devices
• example usage:
  • MicroB web browser on the N900
  • the happy frog game
    • unlike MicroB, this game is open source and even Qt based, so it might just be the question of rewriting the algorithm from C++ to Python
• swirl zoom could be also useful for modRana (map scaling, menu scaling) and other projects