Create a unique texture for each of the different filter types

Right now, the default file icon is used (`editor/textures/icons/file.png`). We'll want a unique icon for each of the different primary filter types:
- Stage (a collection of entities, typically called `map`, `level`, `scene` or `prefab`)
- Mesh (a 3D mesh)
- HTML (a HTML UI file)
- Image (a texture)
- Material
- Shader
- Shader Pipeline
- Render Pass

Start with editor/textures/icons/file.png and trace where the default file icon is selected for primary filter types. Create distinct icons for Stage, Mesh, HTML, Image, Material, Shader, Shader Pipeline, and Render Pass, then verify that each type displays its corresponding icon in the editor.

Written by the indexing model from the issue text.