Only print stacktrace on demand

[PEZ]

The output window quickly gets crowded as it is now when stacktraces are automatically printed. Like it is noted in #819, the important information is often given in the error message itself.

But stacktraces is sometimes very useful, so it is good to be able to print them.

So, let's print these traces on demand instead. Add a command for printing them, and some button in the output window that surfaces the command to the user.