[UI] Graph completion time

[lissyx]

It would be nice if there was some way to have access to the completion time of the task graph. That would mean:

• time it took from running the first task to completing the last one
• only account for execution time, not pending time
• have some way to access that over time and/or split by any means, so we can know what's the trend (e.g., build time/ test time on a specific operating system).

[lotas]

I think this one also can be related to #222, #1305 to provide different timing information about running tasks