Maintain state of task progress animation when workspace tabs are hidden & shown

[MetRonnie]

Closes #1779

Also simplifies use of mitt event bus.

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Not feasible to add automated tests for this
• [x] CHANGES.md entry included if this is a change that can affect users

[oliver-sanders]

(we should push this into the 2.5.0 release)