app, tools: add support for ExcHndl/DrMinGW for Win32 debugging.
The feature already exists in our code and produces backtraces upon a crash into a file. The only difference is that we are now getting the file contents and showing it in our new debug dialog, so that it works similarly on all platform (and therefore making the debug info visible to people, otherwise they would never report, even though the data is generated). The difference with gdb/lldb is that it doesn't allow backtraces at random points (for debugging non-fatal yet bad errors). Also the API has just 2 functions and in particular an ExcHndlInit() but no way to unload the feature. So we don't need the debugging page in Preferences because the switch option would not work. On Windows, the feature will be decided at build time only. Last point: the code is untested on Windows so far. I assume it would work, but there is at least one point I am unsure of: will ExcHndl have already generated the backtrace file when gimpdebug runs? If not, I will have to let gimp die first to be able to get the backtrace.
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