ide-debug-manager: Prefer breakpoint-at-warnings over breakpoint-at-criticals
If both settings breakpoint-at-warnings and breakpoint-at-criticals are enabled, then G_DEBUG=fatal-criticals is used, which doesn't break at warnings. We should use G_DEBUG=fatal-warnings, because that implicitly breaks at criticals. Update the settings schema to more clearly reflect what the settings do.
I considered some sort of binding between the two settings, but the complexity didn't seem worth the benefit (if any).