Add progress reporting across jobs in subprocesses

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 (merged) 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.