app: add GIMP_MESSAGE_BUG_WARNING + GIMP_MESSAGE_BUG_CRITICAL severity.
Since a few commits, I don't generate the traces anymore in errors.c but delay this to gui-message.c and rely on the message severity to decide whether or not generating traces. Unfortunately none of the current severities are properly describing this new type of messages. Even GIMP_MESSAGE_ERROR is used everywhere in our code NOT for actual programming bug, but often for data errors (which are not bugs but proper messages and should obviously not prompt a debug trace).
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