Simplify mutation menu

[MetRonnie]

By figuring out how to use VMenu better I was able to drop a fair amount of cruft.

We were originally using VMenu but through various attempts to get it to behave the way we want (e.g. #927, #1269) I ended up re-implementing some of the behaviour manually (e.g. click-outside behaviour).

It turns out the key to using VMenu and getting it to behave the way we want is not too difficult:

• :target prop - this allows Vuetify to take care of positioning relative to the c-data-interactive element that was clicked on
• :key prop - this allows the menu to re-open when clicking between c-data-interactive elements by using a unique identifier for each element (so Vue knows a different element has been clicked)

Update: also renamed the v-cylc-object directive to v-command-menu as that is more descriptive IMO

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] Tests are included
• [x] CHANGES.md entry not needed as invisible to users

[MetRonnie]

Latest change should make it immune to that kind of thing

[oliver-sanders]

Fixed.

[MetRonnie]

@markgrahamdawson Tagged you for review. If you've not come across the mutation menu code before, it might be best for me to explain how it works before you review

[MetRonnie]

I've also renamed the v-cylc-object directive to v-command-menu as that is more descriptive IMO