Asynchronously check if it is possible to execute the given editing command.
When the operation is finished, callback will be called. You can then call
can_execute_editing_command.end to get the result of the operation.
| this |
a WebView |
| command |
the command to check |
| cancellable |
a Cancellable or null to ignore |
| callback |
a TaskReadyCallback to call when the request is satisfied |
| user_data |
the data to pass to callback function |