[Qgis-psc] Task Manager has landed!

[Nyall Dawson]

Hi all,

Thanks to tons of valuable feedback from the QGIS community and the
generous funding grant from the QGIS organisation, I've just completed
and merged the task manager framework branch.

I'm very keen on feedback from QGIS devs, plugin authors, and users as
they start the play with this new feature. While the bulk of the work
is now complete, I'll continue refining this as we had toward 3.0.

Outstanding tasks include:
- a PR for documentation of the task manager within the python cookbook
- some final UI polish and tweaks
- some blog posts + social media push advertising this feature
- aiding the community with porting their work to task manager

I'd like to once again thank the QGIS org for this grant and everyone
who has donated funds to QGIS which made the grants possible.

(Personally, I think this work is a great demonstration once again of
how healthy the QGIS project is. It's involved donations from end
users, the initiative and leadership of the PSC in creating this grant
program, the new QGIS voting structure + the associated involvement
from local user groups, and a PR with nearly 60 comments from
interested developers in the QGIS community!)

Nyall