Loads an icon asynchronously.
To finish this function, see load_async.end. For the synchronous, blocking version of this function, see load.
| this |
a LoadableIcon. |
| size |
an integer. |
| cancellable |
optional Cancellable object, null to ignore. |
| callback |
a TaskReadyCallback to call when the request is satisfied |
| user_data |
the data to pass to callback function |