- Fix AbstractGraph.destroy() to iterate plugins Map correctly and set null container reference
  - Fix PanningHandler.onDestroy() to call panningManager.destroy()
  - Fix PanningManager.destroy() to call stop() to clear interval timer
  - Fix RubberBandHandler.onDestroy() to remove gestureHandler listener
  - Fix SelectionCellsHandler.onDestroy() to remove listener from selectionModel
  - Add EventSource.destroy() to clear eventListeners, call super.destroy() in children classes
  - Add destroy tests for all modified classes