API: Mode change events

[p-e-w]

Proposed addition:

vis.events.subscribe(vis.events.MODE_CHANGE_PRE, function(new_mode)
  return new_mode
end)

vis.events.subscribe(vis.events.MODE_CHANGE_POST, function(old_mode)
end)

The return value of the MODE_CHANGE_PRE event handler is the mode that will actually be set. Thus to proceed with the requested change, return new_mode, and to cancel the mode switch return vis.mode.