Similar to `setTaskName()`, the string created when calling `subTask()`
must also be stored in the `fTaskName`, rather than in `fMessageText`
field. Otherwise the label of the status line is not cleared after
calling done() on the progress monitor.

Because the text of the sub-task is calculated using the value of
`fTaskName`, the original name of the task needs to be stored in a
separate field to avoid the name of the previous sub-task to be
contained in the next sub-task.

Closes https://github.com/eclipse-platform/eclipse.platform.ui/issues/2762