Show entire expandable call tree in ve.ui.Filibuster

Previously, ve.ui.Filibuster observations showed the stack at state changes, but
the full function call tree could only be observed manually. This code displays
the entire call tree in a rudimentary interface, initially expanded at the state
changes but expandable elsewhere too.

Change-Id: Icfa39084f626ea4ac22cfcccf8e1beacedfc9478