app: only show FATAL errors by default for stable releases.

The debug dialog is awesome and extremely helpful, but I realize it may
be a better default experience on *stable* to raise it only in case of
crashes. CRITICAL are bad and should be fixed, but sometimes their
consequence is actually not visible except for this dialog, and people
on stable builds may prefer not to see this dialog. Also we will likely
get a lot of duplicates for the same bugs once everybody has this by
default, which will be very annoying to attend to, unless we had
automation (which we don't right now).
The option is still available in preferences anyway so people really
interested in helping can activate the option for CRITICAL and even
WARNING anyway, even on stable releases.

app/config/gimpcoreconfig.c

@@ -663,7 +663,7 @@ gimp_core_config_class_init (GimpCoreConfigClass *klass)
 #ifdef GIMP_UNSTABLE
                          GIMP_DEBUG_POLICY_WARNING,
 #else
-                         GIMP_DEBUG_POLICY_CRITICAL,
+                         GIMP_DEBUG_POLICY_FATAL,
 #endif
                          GIMP_PARAM_STATIC_STRINGS);