Event tree building / prep for activity pane tree view

WIP: Not everything here is integrated yet. The main victory is the build tree method, along with prep work. Still to do:
- Pipe event tree into ActivityNode update method
- Build out ActivityTree view
- Install ActivityTree view in event panel

Progress towards #103
Okay cool.  I'll switch that back.  No real value add changing the name; I probably could have skipped  0badf6f.