Use StacktraceSource to provide line number context to Stacktraces

[Evan]

Right now we don't show line numbers with the stack traces that are present on the problem pages. This is important information to have if code has been moved around between releases without actually causing the crash signature to change:

[15:31:55] <diwic_> ev, I'm looking at a stack trace @ errors.ubuntu.com. The error is present in several versions of pulseaudio - from which version are the actual stack trace - line numbers differ between releases?

apport-retrace now provides a StacktraceSource field that we should use. It is described as follows:
"This is a version of Stacktrace with the surrounding code lines (where available) and with local variables removed."