libgimpconfig: Ensure G_PARAM_STATIC_STRINGS

The `GIMP_CONFIG_PROP_*` macros do not automatically set
`G_PARAM_STATIC_STRINGS`, which means each property that uses such a
macro does not have this flag set by default (and almost none set it
themselves). By adding `G_PARAM_STATIC_STRINGS` to
`GIMP_CONFIG_PARAM_FLAGS`, we can prevent this from happening, which
should lead to some unnecessary string copies being prevented.

libgimpconfig/gimpconfig-params.h

@@ -65,6 +65,7 @@ G_BEGIN_DECLS
 
 #define GIMP_CONFIG_PARAM_FLAGS (G_PARAM_READWRITE | \
                                  G_PARAM_CONSTRUCT | \
+                                 G_PARAM_STATIC_STRINGS | \
                                  GIMP_CONFIG_PARAM_SERIALIZE)