Associates some private user_data with a given Object.
To later remove the association call set_user_data with the same key but NULL for the
user_data.
| this |
The object to associate private data with |
| key |
The address of a UserDataKey which provides a unique value with which to index the private data. |
| destroy |
A UserDataDestroyCallback to call if the object is destroyed or if the association is removed by later setting null data for the same key. |
| user_data |
The data to associate with the given object, or null to remove a previous association. |