Asynchronously load a content filter given its identifier.
The filter must have been previously stored using save.
When the operation is finished, callback will be invoked, which then can use load.end to obtain
the resulting filter.
| this | |
| identifier |
a filter identifier |
| cancellable |
a Cancellable or null to ignore |
| callback |
a TaskReadyCallback to call when the load is completed |
| user_data |
the data to pass to the callback function |