Navigation: Allow Spacer Block

[shaunandrews]

Allowing the Spacer block as an inner block of Navigation would let people create visual groupings of links. Something like this:

A horizontal option for the Spacer block could be useful in horizontal navigations, too:

[draganescu]

This issue is currently blocked by the following aspects:

• currently the spacer block only handles vertical spacing
• adding horizontal spacing to the spacer block raises issues with fixed horizontal element sizes and responsiveness

There are also the related issues #23293 and #23294 which could eventually solve this better.

[noisysocks]

I think let's drop this for WP 5.6.

[draganescu]

I would love to have the spacer included in the list of supported blocks in building complex navigation, but it is not a requirement for the editor so I will remove this from the Navigation project.