Displaying the level of parallelism on each node (i.e "Thread reentrance")

[doriancransac]

Right now when we select a tree node, the corresponding threads are highlighted in the top pane. It would be very useful in certain cases to display the thread cardinality on each node (number of distinct threads that go through that node), and allow the use to toggle that number like we're already doing with code line numbers and min count.