… more robust.

GIMP_PARAM_NO_VALIDATE and GIMP_CONFIG_PARAM_DONT_COMPARE were the same
value, most likely because when GIMP_CONFIG_PARAM_DONT_COMPARE got added
(commit c5c807d1), the comment to keep in sync
libgimpbase/gimpparamspecs.h and libgimpconfig/gimpconfig-params.h was
missed.

Instead, since libgimpconfig can include libgimpbase, do the other way
around: first non-GLib param flags are in libgimpbase, then we add a
GIMP_PARAM_FLAG_SHIFT, then we increment from it in libgimpconfig, and
finally we increment from GIMP_CONFIG_PARAM_FLAG_SHIFT if ever we add
more flags in app/ (right now GIMP_SYMMETRY_PARAM_GUI is apparently the
only one, but this may change).