libgimpconfig: fix string deserialization for some cases

In gimp_config_deserialize_fundamental(), we can't use
g_value_set_static_string() because that will in the end pass the
GScanner's temporary scanner->value.v_string to
GObject::set_property(), and depending on set_property()'s
implementation, we might not dup the string (for example objects
created via gimp_config_type_register() will simply use g_value_copy()
and end up with a dangling pointer as a string member).

libgimpconfig/gimpconfig-deserialize.c

@@ -453,7 +453,7 @@ gimp_config_deserialize_fundamental (GValue     *value,
     {
     case G_TYPE_STRING:
       if (scanner_string_utf8_valid (scanner, prop_spec->name))
-        g_value_set_static_string (value, scanner->value.v_string);
+        g_value_set_string (value, scanner->value.v_string);
       else
         return G_TOKEN_NONE;
       break;