Efficiency improvements

Efforts to improve speed of entire rendering pipeline and widget
specific redrawing

* Decent improvements, Ginga can now render HD video (no sound) at 30
  FPS on older hardware (see scripts/example1_video.py).  This
  translates to a slightly speedier feel overall for many operations
  viewing regular scientific files.
* Fixed a bug that gave an error message of
  Callback.py:83 (make_callback) | Error making callback 'field-info':
  'Readout' object has no attribute 'fitsimage'

* Version bump