This was a bit harder since even though we handle fatal signals,
allowing us to do any last action before GIMP crashes, it seems more
memory allocation is not allowed at this time. So creating a dialog or
simply getting the return output of gdb into the main process is not
allowed. What I do instead is running a separate program (gimpdebug)
which will take care of creating the new dialog and running a debugger.
I still use GimpCriticalDialog code from this separate binary, while I
continue to use this widget also within GIMP for non-fatal errors. The
reason why we still want to use it within GIMP is that we can bundle
several non-fatal errors and backtrace this way (fatal errors don't
return anyway) and it's easier to do so when created from the main
process.