Make tree partitioning based on a moveable slider

Partitioning is currently done the same way as in Jalview's old viewer, user clicks in the tree view and from the position of the click the tree is divided into groups. This works but is slightly clumsy and perhaps not immediately intuitive to users. Might be more user friendly to have a slider on the bottom of the tree view that can be dragged by users to the desired partitioning point.