Add progress reporting as a generic string
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.
Some plugins will not be able to have meaningful numbers for reporting progress, but users would still like some way of knowing that the job is not stuck.
To facilitate this, the plugin could periodically report a string that came from the task's output, e.g. the last line of the output when running a BuildElement's commands.
Task description
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Write the appropriate rendering and length-calculating code in the Status renderer. -
Add methods for accessing it -
Implement in a suitable plugin to demonstrate its use (e.g. last line of output for buildelement)