Restore inheritance in StackLinesContent types

When playing around with filtering stack traces, I realised that the content functions related to stack traces/tracebacks were a bit of a mess.

This is an attempt to rationalise it a bit. Instead of having completely separate processing for stack traces and tracebacks, we instead convert stack traces in to traceback-like objects and use the same processing for both.

This should make whatever we decide to do in terms of stack trace processing easier, as we'll only have one place in which we need to make changes.