Snap To Grid is an ActivityListener that decorates and existing
ActivityListener by altering the IlvTimeInterval for the
event to ensure that while the activity is moved or resized that it's times
align with the snap to grid interval.
The snap to grid time interval is controlled via the
gantt.view.snap-to-interval-mins property in the skdviewer.properties
The listener will also listen for MSG_SNAP_ENABLED messages on the SimpleMessageBus
and set the grid enabled/disabled state accordingly.