Add Unit icons and IconRenderBooth

[smix8]

Adds icons for all units and a dev render booth to render them.

Icon Overview

Build Menu

Replaces the single letter buttons with the icons.
Readability should likely be improved further with some labels or better backdrops.

Icon Render Booth

The icons are rendered with a special IconRenderBooth scene in the util folder.

Below the render Viewport each unit has a Node3D with the same name as the unit. The node has its own Camera3D child and whatever custom setup needs to be made visible as well.

When the scene is played it goes through each node, makes it visible and activates the child camera and takes a snapshot image of the unit. The image is stored inside the res://assets/ui/icons/ folder as a png.

Notes

Due the the way that the unit scene file stores their geometry with all kinds of gameplay extras this wasn't very clean. I ended up making the unit geometry unique and discarding everything that got in the camera's way. This naturally lead to some resource duplication now like meshes stored inside the scene file. Imo this should be cleaned up and improved on the unit level by having the visible unit geometry like meshes split from the rest of the unit. This is already partially done for the structure-geometries.

@Scony the "grid" texture that I ended up using for the ground is from the kenney prototype textures pack.
https://www.kenney.nl/assets/prototype-textures

[Scony]

Looks perfect, thanks!