Add progress reporting across jobs in subprocesses

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1092)
In GitLab by [[Gitlab user @jonathanmaw]](https://gitlab.com/jonathanmaw) on Jul 26, 2019, 14:22

## Background

As discussed in https://mail.gnome.org/archives/buildstream-list/2019-July/msg00020.html (an E-mail in response to #1029), it would be beneficial to be able to report the progress through a task, so that users know whether a task is hanging or making progress.

To facilitate this, !1482 adds in numerical progress reporting for the main process.

## Task description

* [ ] Pass the Task object created by the scheduler into the Element that is running
* [ ] Fix any IPC that is preventing notifications in the plugin reaching the frontend
* [ ] Add methods in Element to use the Task to report progress
* [ ] Implement progress reporting for whichever plugins seem appropriate

## Acceptance Criteria

It is possible to use function calls in the element plugins' implementations to report progress information that appears in the status display.

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[//]: # (To review information about possible relevant labels for this issue please view the list of labels: https://gitlab.com/BuildStream/buildstream/labels)

Start with the original GitLab issue and the scheduler, Element, IPC, plugin, and status-display areas named in the task description. Trace how the scheduler-created Task reaches an Element and where plugin notifications cross IPC; the work is done when plugin function calls report progress that appears in the status display.

Written by the indexing model from the issue text.