Describe the project you are working on

Apply to any project, but I am specifically working on a 3D top down shooter.
This PR should come after this one is merged I think: godotengine/godot#91874
It follows up on the work started in godotengine/godot#87623.

Describe the problem or limitation you are having in your project

Inconsitency in whether we get a tooltip and where this tooltip appears when drag-and-drop something in 2D/3D Viewport.
The problem is discoverability and visibility of the drag-and-drop behaviour modifiers (default, ALT, SHIFT).

Drag-and-drop for images/textures in 2D View: Tooltip appears at the top with a nice highlight of the tool name:

Drag-and-drop for mesh in 3D View: Default, ALT and SHIFT behaviours aren't explained for 3D users.

Drag-and-drop for material in 3D View: Tooltip appears at the bottom (easy to miss):

Also note how the white text can be unreadable when a bright object is behind it.

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

I would like to basically replicate the solution existing for the 2D Viewport into the 3D Viewport, to make it consistent.

Hence, when drag-and-drop a mesh or a material in 3D View, a tooltip will appear at the top left, with the tool name highlighted, and the decscription explaining the diffferent behaviours (default, ALT, SHIFT).

I might explore the possibility to add a semi-transparent box around the tooltip to make the text more visible in cases where there is a white object behind the text.

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

• I will probably reuse the existing preview_material_label_desc and preview_material_label, but rename it something like drop_tooltip_label_desc and drop_tooltip_label
• Use it for both material and mesh tooltip display

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

It is often used.

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

It is about feature discoverability and consistency.