Drmingw already added its own exception handler which generates crash
traces in a text file, for plug-ins as well. This additional handler is
run after Drmingw handler and allows us to do things on our own, and in
particular we could display the content of the debug traces.

Right now it simply prints these to stderr, which actually won't be of
much use on Win32, first because the console is deactivated on stable
releases, also because after tests, it doesn't look like even running
GIMP from cmd outputs to console either.

We currently don't use the same debug dialog as the core on purpose,
because we don't want everyone to send us traces for every unmaintained
third party plug-ins out there. But we should definitely allow easier
trace possibilities at some point, first to improve/debug our own core
plug-ins, and also to help third party plug-in developers!
So this commit is not making visible changes yet but is actually a first
step towards these debugging goals.