The function list is now filtered by category by default, and sorting improved to break ties better.

Since the butterfly trees get massive, I had to start expanding the call trees on demand. This is implemented by keeping track of all ticks that cross given tree node (+ position where they crosses it). This seems to work quite well - in subsequent CL, I am planning to flip the bottom-up and top-down trees to use this.

Review-Url: https://codereview.chromium.org/2730293002
Cr-Commit-Position: refs/heads/master@{#43602}