Quick smart items from assets

I can drag a 3D model from the Local Assets tab into my scene, and this creates an item with a GLTF and Transform component.

Would be great if we could offer a similar experience from other kinds of assets.

- If I drag an image file into the scene, I instance an entity with a plainshape that uses that image as a texture.
- If I drag a sound file, I instance an invisible item, an ambient sound smart item (#72) with an audio source that uses this file
- If I drag a video, I instance a video smart item that uses this video as the source