Asset Shelf - Add thumbnail quick size toggles for consistency

[Draise14]

Current

Suggested

Make the quick toggles to set asset shelf thumbnail size standards into the overlay settings of the asset shelf.

Remember.. this is only relevant in the Pose Mode of an armature when you have created a pose as an asset atm.

This overlay panel also needs to be documented.

Details

SHA-1: 05b125f

• Asset Shelf: Add default preview size option

Allow changing the size of previews in a new asset shelf
when registering a new asset shelf type.

Pull Request: https://projects.blender.org/blender/blender/pulls/121358