Navigation Editor: Add visual cues for the menu and page list items

[javierarce]

I've noticed that the interface lacks some visual cues (ie: highlights) in a couple of places:

• When the user hovers the main menu after it has been created
• When the user hovers the main menu in general
• When the user hovers a list of pages

This lack of feedback from the UI makes the discoverability of the edit options a bit difficult (for example, I had some trouble discovering the main sidebar menu when I started working on this project).

Here are some videos showcasing the problem:

Initial.placeholder.mov

Main.navigation.mov

List.of.pages.mov

Proposed solution

I'd highlight the border of the main menu in blue and the list of pages in grey.

[javierarce]

When we fix this issue about the placeholder state, we'll also solve one of the problems mentioned here.

[javierarce]

I'm closing this issue since the navigation screen is no longer actively developed.

Related: #43620.