Mouse scroll wheel zooming! =o)

 * ticket:252
 * MussaWindow & MussaAlignedWindows now zoom when using the mouse wheel.
 * SequenceBrowser emits mouseWheelZoom(double) when mouse whell use
   is detected.
 * SequenceBrowserWidget perculates the mouseWheelZoom(double) signal upward.
   * If you need to act on new mouse wheel zoom, you can connect to the
     signal at the SequenceBrowserWidget level.
 * Zoom automatically adjusts increment when switching form 1.0, 0.1, 0.01
   levels for smooth zooming.