Fixes Issue 981

Allow user to turn off gesture-based control.

Also, consolidate so that gestures could be used by other subclasses
of K9Activity.  Probably should be made usable to K9ListActivity, too,
by making MyGestureDetector its own class and make it used by
K9Activity and K9ListActivity, and have the Activities implement a
callback interface for onNext and onPrevious.