Output press/release/click events and persistent control state

- Buttons now output press/release/click one-shot events.
  The click event includes the count of sequential fast-clicks
  that can be used to support double-click, triple-click, etc.
- The value of a button event is the time at which the
  event occured.
- The value of an axis event is the value of the axis.
- The state of a control is now written to memory a persistent
  value so that parts can use them as inputs.  This
  allows a part to use one control's value as a modifier to
  another control's event.  It also allows a part to use an control's
  state without having to listen for events and maintain the
  control's state itself.