Added functionality for Agenda view 'selecting' event

I had  a problem as i needed to update a popup when a user started to select cells (drag select) however the update only occurred after the mouseup event (end of drag).

I want this to occur whilst the user was dragging hence a 'selecting' event.

(i have only tested this on the Agenda view)

There might need to be tweaks on the other views - i am not sure how the internal workings of selection manager work.