We don't want an infinite number of traces because it takes some time to
get. Until now I was keeping track of traces in app/errors.c, but that
was very sucky because then I was limiting traces per session. Instead
save them as a variable of a GimpCriticalDialog instance. Therefore only
generate the traces for WARNING/CRITICAL at the last second, when
calling the dialog.
When too many traces are displayed, just fallback to just add error
messages only. But then even errors without traces can be time-consuming
(if you have dozens of thousands of errors in a few seconds, as I had
the other day, updating the dialog for all of them would just freeze the
whole application for a long time).
So also keep track of errors as well and as last fallback, just send the
remaining errors to the stderr.