Add support to enable/disable SmudgeKit at will

Submitted for your consideration is a patch that adds the capability to enable/disable SmudgeKit at will.

There are a few apps that offer a "Demo Mode" that can be turned on/off in preferences that, when enabled, shows the touch points on screen like SmudgeKit does.  (I believe the Omni* apps do this)   This can be useful, e.g. during normal use, you wouldn't want the touch points being shown, however if you are recording a screencast or demoing an app to people using a projector or Apple TV, then you would want to show the touch points.  So having this a togglable setting is useful I think.