timeline: cannot set an icon instead of grippers

For zero duration events in a timeline it is not possible to have an icon instead of the odd-looking "grippers". Having event specific icons would mean the labels are no longer essential, which would reduce clutter in the timeline display without losing the semantic meaning of each event.

SmartGWT version: SNAPSHOT_v10.0d_2014-04-27/Pro Deployment (built 2014-04-27)