Brushing enhancments

[tdudgeon]

Suggest 2 enhancements that will allow better use of brushing. The scenario is that the result of a brush is propagated on to another component as a "filter" and want to avoid unnecessary communications overhead.

1. add a "brushend" event that allows selection to be propagated only after brushing is complete.
2. provide a mechanism to determine if nothing is brushed. Currently the "brush" event fires with all data when bruses are cleared. Would be more efficient to allow to determine if brush is empty so that filter can be cleared rather than set to all data elements.