Editor grid visibility should be allowed to be toggled per viewport

[ryevdokimov]

Describe the project you are working on

N/A

Describe the problem or limitation you are having in your project

I'd like to not see the grid in one or more of my viewports in the instance that I am making changes and want to have a viewport that reflects those changes without distractions from the grid.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add an option in the perspective popup to allow the grid to be toggled per viewport.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Split out the code from Node3DEditor class, and have it render as part of the Node3DEditorViewport class. In order to not break compatibility, any global grid visibility will loop through all viewports and toggle visibility simultaneously.

If this enhancement will not be used often, can it be worked around with a few lines of script?

Not that I'm aware of.

Is there a reason why this should be core and not an add-on in the asset library?

I believe it's a good usability feature, that can't be implemented as an add-on.

[ryevdokimov]

Related to: godotengine/godot#87793

[akien-mga]

Implemented by godotengine/godot#88584.