app: new error dialog to backtrace and encourage people to report bugs.
GIMP will now try to get a backtrace (on Unix machines only for now, using g_on_error_stack_trace(); for Windows, we will likely have to look into DrMinGW). This is now applied to CRITICAL errors only, which usually means major bugs but are currently mostly hidden unless you run GIMP in terminal. We limit to 3 backtraces, because many CRITICAL typically get into domino effect and cause more CRITICALs (for instance when a g_return*_if_fail() returns too early).
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