Enable a new "slide over" accessory view controller presentation style.

AppMenuViewController Class:
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Make AppMenuViewControllerTransitionSlideOver available as a choice for
transition style. This transition moves the accessory view over the main
view.

Change the model for storing and altering constraints. The old model was
to store the constraints that would need to change and replace/alter
them when transitioning. With the new model, constraints change enough
that it is simpler to store all constraints for a particular accessory
view controller in an array and remove all the constraints and start
from scratch when changing the transition style. The constraint that is
altered to animate the transition is contained in the array, but is also
referenced by an ivar to simplify changing the constant.

Also change topTravel, etc. properties to readonly. Changing them
requires changes to the layout constraints that haven't been implemented
yet.

Testing/Demo Harness Changes:
------------------------------
Add "Slide Over" as a choice in the "Animation Curve" picker and use enum values for the number of choices available in both pickers. Remove unused UIPickerView delegate function "-pickerView:didSelectRow:inComponent:". Returned "cancelsTouchesInView" property of UIGestureRecognizers to default "YES" value.

Future Work:
-------------
* change topTravel, etc. properties to readwrite and add code to update
  views while hidden and displayed.
* enable AppMenuViewControllerTransitionPushOver transition type.