New pie wedgeEventsEnable/Disable implementations

- The wedgeEventsEnable and wedgeEventsDisable methods now increment and
  decrement a counter.  When the counter is 0, mouse events are enabled,
  otherwise they are disabled.  As a result, multiple calls to
  wedgeEventsDisable require a matching number of calls to
  wedgeEventsEnable to turn wedge mouse events back on.  This is useful if
  multiple animations will happen in succession and the goal is to
  disable mouse througouth the entire transaction.
- Simplified generateId method in core.