update Viewer.save_animation() for selected steps or times

- update Viewer.get_animation_info(), .encode_animation()
- remove FileSource.get_step_range()
remove animate, anim_format arguments from Viewer.__init__()

- update view_file(), postproc.py
- update Viewer.call_mlab(), make_animation()
update Viewer, ViewerGUI for saving snapshots in given ranges of steps/times

- update Viewer.call_mlab()
- update make_animation()
- update SetStep:
  - new .seq_* traits
  - new .init_seq_selection(), ._seq_n_step_changed(), ._seq_dt_changed()
- new ClosingHandler
- update ViewerGUI:
  - custom quit button
  - remove button_make_animation, button_make_snapshots buttons
  - new button_make_animation_steps, button_make_snapshots_steps
    button_make_animation_times, button_make_snapshots_times buttons + their
    handlers