Make pressing 'b' on a set_trace() line skip it

All that remains to do is to show this in the UI.
Highlight set_trace() like a breakpoint when its status changes

Unfortunately, I'm not sure how to make it highlight initially, because I'm
not sure how to communicate that information from the debugger to the UI.
Highlight set_trace() on first call

This only works if it is the first time set_trace() has been called in the
file. Further set_trace() calls are not highlighted.