Continue work on the Stacktrace View.

It should show you data and context for each frame within the trace, and should:

- [x] Show a few lines of context around each stack frame's line.
- [x] Jump to any given frame's location in source.
- [x] Collapse each frame.
- [x] Make the current trace the "active" one (done automatically during 1) or "deactivate" it.