[ Navigation screen ] Let's have only one loading indicator

[draganescu]

Incoming after #23033 will land we'll have two spinners that show up:

• one spinner for loading the list of available menus
• one spinner for loading the menu items in the selected menu

Ideally we'd only want one spinner, as multiple loading indicators tend to crowd the visual information highway :)

The idea, discussed with @adamziel is to wrap the page in another component which would know if any of it's children still is in a loading state.

Another option is to have one of those nice loading states where we have a skeleton of gray boxes until we can actually build the page. But this pattern should exist elsewhere as well I'd think.

[adamziel]

I'll spin a proposal PR once #23033 lands to show what I think would be the cleanest solution here - a picture says a thousand words :-)

[shaunandrews]

With the newly updated navigation screen, we now have a single spinner: