LibOverride: Fix 'overridable' flag desapearing when setting IDProp from py code.

Do not see any reason not to copy over the flag of the old, existing
IDProp to the new one when assigning (e.g. `C.object['prop'] = 0.5`
would nuke that IDProp flag).

source/blender/python/generic/idprop_py_api.c

@@ -688,6 +688,7 @@ bool BPy_IDProperty_Map_ValidateAndCreate(PyObject *name_obj, IDProperty *group,
       /* Preserve prev/next links!!! See T42593. */
       prop->prev = prop_exist->prev;
       prop->next = prop_exist->next;
+      prop->flag = prop_exist->flag;
 
       IDP_FreePropertyContent(prop_exist);
       *prop_exist = *prop;