[6.x] Show asset editor saving state on the Save button

[jaygeorge]

When you save an asset in the asset editor, a dark loading overlay covers the whole editor until the save finishes. It shows up as a brief black screen before the modal closes, which is jarring for something as quick as updating alt text.

This removes the overlay and shows a loading spinner on the Save button instead, so the image and fields stay visible while saving.

The loading overlay shown while the editor first loads an asset is unchanged.

How to Reproduce

1. Open an entry with an Assets field and click an image to open the asset editor
2. Change the alt text and click Save
3. On 6.x the editor flashes black before closing; on this branch the Save button shows a spinner instead