Make node-specific menus/buttons more obvious

[trollusk]

Describe the project you are working on

Action RPG.

Describe the problem or limitation you are having in your project

Some nodes, when clicked on in the SceneTree, make context-specific menus or buttons appear in the toolbar above the main editor viewport (to the right of the "Transform" and "View" menus in the 3D view).

Examples are MeshInstance3D (creates a Mesh menu) and Skeleton3D (creates a Skeleton3D menu and several buttons).

These menus/buttons are very unobtrusive, to the point that I generally don't notice they are there unless I'm specifically reminded to look there by a tutorial etc.

There are other Nodes that make node-specific controls appear in more obvious ways. Eg when Camera3D is clicked, the "toggle preview" button appears within the editor viewport, where it is much more likely to be noticed.

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

Make the node-specific controls appear within the main editor viewport, similar to the Camera "toggle preview" button.

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

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

No, this is a basic editor UI enhancement.

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

This is about editor usability.

[Calinou]

The issue with displaying context menu options on top of a 3D viewport is that you can have multiple viewports in split view. The context menu option is global to all viewports; it's not scoped to a specific viewport. We should avoid duplicating an UI option if it ends up doing the same thing regardless of the viewport you click it on.

[KoBeWi]

Duplicate of #3550 (and #14)