toolbar: add n-window-selector

[oliver-sanders]

• Closes #864
• Add a selector for switching the n-window extent between 0 & 3 inclusive.
• The n-window value appears to be preserved when navigating back to the workflow.

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] Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• [ ] Tests are included - seems to be hard to test meaningfully without true e2e tests (i.e. with live workflows).
• [x] CHANGES.md entry included if this is a change that can affect users
• [x] docs issue - https://github.com/cylc/cylc-doc/issues/719
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

@MetRonnie, this setting is preserved when you navigate back, however, I have not done anything to achieve this. Should it be implemented differently or is this ok? (note the n=1 default comes from cylc-flow not cylc-ui)

[MetRonnie]

It's preserved because it's querying it, right?

[oliver-sanders]

Derp, yes, it's getting the n-window extent from the returned data :facepalm:

[oliver-sanders]

As a true end-to-end feature (i.e. flow-uis-ui-uis-flow) it's a right pain to test. I had a look a testing, at the unit test level you end up testing vue watchers and at the e2e level, you end up testing the Vuetify Select API.